That seems to be a mistake that’s been made so much it became canon. In TNG, betazoids were full telepaths, much more so than vulcans, and only Deanna (and some other betazoid/human introduced for one episode later), were ‘just’ empaths. But writers who aren’t Trek nerds have conflated Deanna’s unique limitations with the entire species at times. It’s been pretty inconsistent. When introduced, Ferengi’s four lobed brains made them immune to Deanna’s powers, but by DS9, that was no longer true either, as Lwaxana, IIRC, read Quarks brain and said he was a revolting little troll for thinking such thoughts.
OTOH, Lwaxana might not be the most reliable source, and I could be wildly misremembering.