Getting a prepaid unregistered SIM is no longer possible in an increasing number of countries. I think most of Europe requires identification to buy a prepaid SIM.
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.
The UK notably doesn't, and most providers are still doing "roam like at home" with the EU despite the UK having left.
Thankfully this is still possible in the US... for now.
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.