That answer is wrong wrt the application fee - it applies to the tuition fee for public universities, ie., the instate resident vs. Out of state applicant differential, not to the application fee. US citizens who live abroad are almost never “instate” for a public university. However neither OOS nor citizens living internationally have to fall under state residency requirements to avoid being qualified as “international”.
Please reach out to them and see if they can correct this as it is misinformation.
However for private universities and most importantly for the application fee they’re US citizens so automatically fall under “domestic applicant”.
CommonApp does it automatically.
Most universities specify that “international” means “if you need a visa”.