21st Century: Tequila, white Creme de Cacoa, Lemon, absinthe rinse. Sorry, but meh. I added a couple of dashes of Xocalatl Mole bitters, which helped a little. I sort of liked it at the start, but was glad when the glass was empty.
Pretty much the same way I felt about it. I enjoyed the 20th well enough and the offshoot 19th (bourbon, dubonnet rouge, creme de cacao, lemon juice) even more (I think, it's been a while) but something about the 21st didn't work as well for me. My lack of experience left me unable to figure out exactly what that something was but it didn't feel like it continued the chain started by the other two and didn't completely agree with me even as a standalone outside of the "century" theme.
I'm pretty sure that the 19th Century, as originally formulated by Brian Miller at Pegu Club, contained bourbon,
Lillet Rouge, white creme de cacao and lemon juice. Not that Dubonnet Rouge (or even Bonal) wouldn't work plenty well.
But your point is well made about the fact that this tequila drink breaks the model, which in its loosest interpretation is: base spirit, white creme de cacao,
quinquina, sour citrus. The two elements that hold these drinks together, in my opinion, is the white creme de cacao and the quinquina. Without that thread of chocolate and the bitter quinine finish, it just doesn't taste like a "century" drink to me. The connection is further muddled by the incorporation of non-cannonical absinthe. I'm not saying it isn't a good drink, just that it's a bit "one of these things is not like the other" alongside the 20th Century and the 19th Century. In the case of the 19th Century, the use of a stronger-tasting and more tannic red quinquina made sense together with whiskey. With tequila, I'd think that a return to a white quinquina would be in order -- perhaps an Americano would be interesting. I'd be interested to see what tequila, white creme de cacao, Cocchi Americano and lime juice would taste like.