Public figures, which would absolutely include King Charles III are fair game in P&E - in much the same way P&E threads about Elon Musk offering to buy votes in the past election are. Sure, Elon wasn’t (at the time) an elected or appointed official of any government, but he was a public figure who was involved in politics.
Which forum a thread should be in is often determined by the intent of the OP. We have plenty of gossipy threads about the UK Royal Family that aren’t in P&E as well. The specific thread is probably less tightly focused than I’d like for P&E, but does go into actual P&E details quite often.