It's much easier and safer to just assume someone who's in Europe is a resident, rather than figuring out if they really are.
GDPR only applies to EU residents, yes, but not if they're on ex. holiday outside of EU.
Say, a EU citizen is on holiday in The U.S.
In such case the EU citizen is not protected by GDPR.
While this is an interesting way to interpret it, it's likely that the law may be clarified in the future to state that if at the time of collecting their data the user is in the EU, the protections shall apply to said data regardless of where the user is now.