I agree on the bigly-level stuff (it's very similar to the facile jokes about George W. Bush being dumb) but this quote specifically is clear cut. Of course Trump is not calling Mexico/Mexicans rapists (though he does disparage Mexico shortly before it) but he was calling most Mexican-Americans criminals and rapists: since the ones Mexico sends into the US are "their rapists" and Mexico isn't "sending their best", it stands to reason that the resulting Mexican-American population, for Trump, is overwhelmingly comprised of criminals and rapists.
You're still parsing it incorrectly even if this speech wasn't about the wall and illegal immigration.
"their rapists" only means that some subset are rapists because it's a category included with others. There is no hint that all of the bad ones he is referring to are rapists.
