I have not had an issue buying a prepaid sim in eastern europe AND EU is a "free roaming" zone, so if it works in the Eastern parts, it's going to work everywhere else as well.

Thankfully this is still possible in the US... for now.

The UK notably doesn't, and most providers are still doing "roam like at home" with the EU despite the UK having left.

Have you ever heard of LycaMobile?

Most likely they have heard of the MVNO/"service reseller" LycaMobile, but how does that respond to their comment at all? LycaMobile still require you to register your ID with them in countries that have that rule.