This question relates to algorithms and the definition Facebook has set up for "EU User". What parameters have Facebook's legal team considered adequate to label an account belonging to an EU user? It is difficult to say.

They will probably look into multiple factors like phone number location and GPS-backed data to identify the true location of an EU user. Now, the interesting part is that if you use a VPN quite frequently, you will notice that IP geolocation data is treated as a universal truth on cookieless interactions. Facebook will likely have a significant amount of data beyond IP geolocation data to infer the location information of their user. But they are likely not going to do that as they launch this new feature. So, it all comes down to how seriously Facebook wants or is required to identify an EU user.