> I suspect "Oh that's wonderful" is not meant to be literally interpreted as him thinking it's wonderful.
Yes he abso-fucking-lutely means it's "wonderful", only not in the way you apparently misinterpreted it.
> I also don't think he actually sees those military forces as true peacekeepers either.
No, of course not. Where on Earth do you even get such an idea from? You're literally the first person I've seen even entertaining that hypothesis.
> To me that sounds like he's mocking the idea that the force is actually for peacekeeping because it is the size and composition of an army.
Yes, of course he's mocking that -- in admiration of the sheer audacity of using such a transparent lie. That is what's so "wonderful" about it, in his world -- it's exactly his own MO, but on a scale he has never got close to himself. He's (possibly literally) pissing his pants with glee over Putler's chutzpah.
And that's why you can clearly hear from his voice that "We should do that on the Mexican border" was not sarcasm: It's a 100% serious recommendation that the USA should do the same, unilaterally declare some piece of Mexico "independent" and then go in and "peacekeep" it for themselves.