Mike!
Official Ponylandistanian National Treasure. Re
It ruins a joke to explain it (and I only intentionally do that with dad jokes), but the joke is already ruined, so here goes:
The point is not about whether other people feel the same way. The point is that, grammatically, you've identified "I" (singular) as being "Americans" (plural). It would be like saying "I am Americans". It's this awkward mix of singular and plural that I was making fun of, not the issue of whether your position is shared.
You're just being a singleist. If he wants to self identify as plural, they have every right to.