A few thoughts ...
-Probably the chef pre-hydrates the xanthan not because it's hard to hydrate the stuff, but because it takes a while for it to fully hydrate. A blender or homogenizer will disperse it in a hurry, but it can still take a couple of hours to fully hydrate. This means that you can easily overshoot with the concentration. Pre-hydrating would take the guess work and the timing out of the picture.
-I'm not convinced he told you the full story. Anything thickened to that consistency with pure xanthan gum would be disgusting. It would basically have the consistency and mouth feel of snot. My guess is that that stuff (whatever it is) was naturally thick, based on whatever was pureed, or else it had some other ingredient that acted as a hydrocolloid. The xanthan could then be used to add a bit of viscosity, cling, and stability.
-If you have to get a thin liquid to that viscosity, a couple of suggestions:
-a 3:1ratio of lambda carrageenan and xanthan. Use at a concentration 0.4% to 0.6%. Doesn't have to be cooked, but should be dispersed with a blender to prevent clumping.
-a 10:1 ratio of arrowroot starch and xanthan. Use at a concentration of 0.5% to 1%. Needs to be cooked to hydrate. But pretty easy to disperse just by making a slurry and whisking.
-Maybe the green is chlorophyl. Or maybe it's a Shamrock Shake™®©
