Yeah, no kidding. Of course they want you to prove you're a EU citizen, because they want to make as little effort as possible.
I don't consider that reasonable. Data portability should be a right. You shouldn't have to jump through hoops to exercise that right, and companies shouldn't be asking you "can you prove beyond a doubt that we're legally obligated to give you your data" before doing so.
I agree, I would like them to offer this service to everyone, but the reason they ask for an EU ID is not "emails are to easy, have them suffer" but more likely "we really have not set up a process for this so we will not do this unless forced by the law"
The solution to this is to have their own government impement a GDPR-like policy.
I don't consider that reasonable. Data portability should be a right. You shouldn't have to jump through hoops to exercise that right, and companies shouldn't be asking you "can you prove beyond a doubt that we're legally obligated to give you your data" before doing so.