I'd agree with this as well, but then again the whole "Americano= somebody from every New World country, not just the USA" attitude present in many Latin American countries really sticks in my craw, so YMMV.
I'm Mexican, so I grew up with that "America" is a continent not a country attitude. However I grew out of it 'cause its pointless.
However in TTL, it is easy to see how norteamericano would be the common monkier for Americans, since there is a "Southern America".
Sudamericano is more conflicting because there is a Southern America and a South America, and they mean to different things. Using Iberoamerica for the southern continent is the only solution I see.
And once again the attitudes in TTL about this would be different from OTL's so maybe there isn't that "america" is a continent identity crisis here. Or alternatively it is worse.
I also have to wonder whether there's a greater sense of "provincial citizenship" to parallel OTL's States Rights attitude; in other words, a tendency to identify more strongly as an Indianan/Georgian/Abionian/etc. in addition to, or instead of, a Southron. Hopefully that made sense .
I'm going to guess this is likely the case within the Dominion, at least up until the Global War. Then an "Southron identity" has likely taken over. The degree also likely varies from region to region.