I'm just guessing, but if he's from Canada he could be described as (North) American and perhaps he wanted to make that distinction. USAian is hardly a slur, it's entirely different from your Dumbocrats/Republitards example.
Clearly many people here do not view "USAian" as an offensive or derogatory term. I'll try to explain why I believe it to be inappropriate and hope I can do so in a polite and effective enough manner to avoid more downvotes.
"USAian" reads to me as someone who, under the guise of disambiguation, is intentionally being diminutive. Very rarely does a situation occur in which "American" as a term for the people from the US actually result in confusion versus the entire continent. It seems most likely that the speaker is using it to intentionally point out the unimportance of the USA to counter an implied American self-importance.
You're using a term for a group of people that they would never use for themselves, which is a strong indicator it may not be a desirable label. Further, given common themes in current global culture, it is very easy for the term to be taken as an intentional insult, as if the speaker is "putting them in their place".
Whenever I see something like "USAian" I assume the speaker views Americans as self-centered, imperialistic, unworldly or similar and is using the term to intentionally remind the reader of the significance of the rest of the Americas, with the implication they need this reminding.
But, based on the responses, maybe I'm an outlier. Hopefully the above clarified my original response.
(On reflection, how does "US American" strike you? This seems analagous to "South Korean" or "Mainland Chinese")
Go strike up a conversation with anyone in the world and refer the people from the New World as 'Americans'. That person will assume you are referring to US citizens. Now, should the issue be forced somehow you will have to explain that, in fact, you were referring to folks from the New World all along, not so-called US Americans specifically. That person will then probably roll their eyes a bit.
So it's a moot point. You're trying to resolve a confusing situation that has virtually no chance of ever actually occurring, where in fact your solution would actually introduce confusion.
The standard English nomenclature for residents of the USA (which I belive isn't so much of an issue in other languages) is confusing.
But of course language is never wrong, it's just what people speak.
Americans have always called themselves americans: before there was a USA these were the american colonies. If you wanted to distinguish yourself from the continental subjects you called yourself an "american".
It's only in the last half century of hegemony that people started freaking out and thinking that somehow the term was an attempt to usurp ownership of the continent or whatever.