Is it relevant how much? They have admitted they know their business is partly based on slave labour. Even if that is 0,01% or less, which I doubt, it is is disgusting. The fact that CP don't know or seem to care is disgusting.
Although you have rightly said there is no excuse, at the same you seem to be working pretty hard to get them off the hook. Who else is to blame? As I said, the slave operators only do so because CP pay them. Cut off the funding and it will end. But so will CP's business and we can't have that, can we?
But this is all a huge distraction from the real issue.
The fact remains that at least some of the shrimp you can buy in the aforementioned stores is produced using slave labour to some extent. The company conducting this business knows it and admits it, but attempts to excuse its use of slave labour by resorting to meaningless corporate babble. Where is the "OK. Busted! We'll stop it"? Of course, nowhere.
The focus on CP Foods is misguided and overly narrow. I'm not trying to get CP off the hook for anything, but the problem extends far beyond that particular firm and their practices. Even if CP displayed due diligence in sourcing their products (maybe they did, maybe they didn't) it would be extremely difficult if not impossible to ensure that their products were free of slavery or forced labor. Let's look at the broader context.
The largest and most systematic survey of the Thai fishing industry's labor practices is this study jointly conducted by the International Labour Organization and the Asian Research Center for Migration. The study interviewed 596 fisherman and found that 17% of respondents were working against their will and unable to leave because of threats of penalties (either physical violence, financial penalties, or governmental penalties for undocumented/illegal migrant workers). 5.4% did not enter the fishing sector voluntarily but were forced into it by manipulative handlers. Coercive and deceptive hiring practices are pervasive, especially amongst long-haul fishermen who journey into international waters. Forced labor and human trafficking were found to disproportionally impact migrant workers who immigrate from nearby Myanmar and Cambodia (which are both plagued by economic stagnation and political turmoil). These displaced workers move to Thailand to seek economic opportunities that do not exist in their homeland, but are unable to enter Thailand legally due to strict laws governing immigration. They are often trafficked into the country by exploitative guides who promise them jobs but deliver forced labor or outright slavery. Even those who are not trafficked in this way must sell their labor on the black market where there is no legal regulation. This offers still more opportunities for exploitation and coercive labor practices. Even domestic Thai workers are subjected to exploitative practices, largely because most workers (94%!!!!) had no written contract, and thus no way to estimate their wages, their occupational responsibilities, or the length of time they would be required to work. The lack of a contract also makes it difficult or impossible for exploited workers to challenge their employers in court. What's worse, widespread official corruption by government employees who are supposed to regulate this market and address worker complaints creates an atmosphere of hopelessness for victims of trafficking and forced labor.
The upshot of this? The Thai fishing industry is systematically tainted by exploitation, corruption, and deceptive labor practices. You might suggest that companies simply stop buying fish from boats guilty of these sorts of injustice, but the supply chain for the industry is so muddied and obscured that it's this isn't feasible. Fish from different boats are mixed at sea, and are often mixed again at processing plants and ports. All this mixing happens before the larger companies even come into the picture to buy these products.
Max Tunon (one of the authors of the ILO study) claims that it is "close to impossible" to disentangle Thailand's fish supply chains. This means that it's close to impossible to ensure that your products are exploitation/slavery free if you're buying fish meal in Thailand. If you're buying fishmeal in a commodity market where almost 1 in 5 fishermen are working against their will, there's not much that can feasibly be done. When CP Foods claims that they don't have visibility regarding the ultimate source of fish that their suppliers use to make fish meal, it's not "meaningless corporate babble." It's true. And even NGO non-profit worker's advocacy groups like the ILO acknowledge that it's true.
There's no simple way to say "OK. Busted! We'll stop it!". And if you can come up with one, I'm sure that CP Foods, the ILO, and tens of thousands of Thai fishermen will be interested in what you have to say.
Thailand is the world's third largest exporter of seafood, and the United States is the number one destination for exported Thai fish. 17% of fishermen in Thailand are working against their will. This problem is way bigger than the shrimp at Walmart and Aldi. Simply not buying foods from CP Foods will not solve the problem. Solving the problem will require liberalized immigration policies in Thailand, better enforcement (or just the existence) of contracts for fishermen, active governmental regulation/inspections, and a reduction of official corruption. It will require a coordinated effort by the Thai fish industry to render their supply chain more transparent and to improve the practices of the firms operating in the market. It will require a lot. Until things improve, don't buy seafood from Thailand. Don't just boycott a single firm; boycott their entire industry.