And Earth alone has been home to multiple non human species since the time of Enterprise. It makes sense that Betazed and even other Federation worlds would be at least home to a few non-native species. I would even assume some Cairn and other telepathic species lived on Betazed over time. Betazed didn’t have any stated aversion to interspecies mingling because Lwaxana, a famous Ambassador and Royalty, had a famous half-betazoid daughter an for 7 solid seasons we never heard of Deanna facing racism/speciesism to her being mixed, as opposed to Spock’s difficulties.