I would argue against you by pointing out that plenty of users here talk about "an italian" or "a brit" or whatever, but then that would require me to recognize the italians and british as people.
It's not about the adjective, it's about the suffix - adjectives ending in -n are considered normal (an american, a german, a paraguayan), but adjectives ending in -ese are considered to sound weird and need a "person" adding afterwards (a chinese, a congolese, a portuguese).
For once I'm pretty sure this isn't a racism thing, just an "the english language is a fucking mess made up of more exceptions than rules" thing.
I think the weirdness comes from the fact that words ending in -ese are both singular and plural, while you need to add an -s as a suffix for other nationalities.
"That chinese is riding a bike" / "the Chinese are riding bikes" vs "that German is riding a bike" / "the Germans are riding bikes"
There we go, you see it in "a Dutch" and "an Irish" too, because they're singular and plural, and don't see it in stuff like "a Pole" or "a Scot" because they have a different plural form.
Guess I was wrong, for once it is an actual rule.
Who knows if its an actual rule though? I think -ese can be singular as well as plural, so it should be fine, but it sounds wack. Maybe it's a rule like "I before E, except after C (weird!)"