Fiilonof is more than just vibrations. They're also something of a psychic symbiote. Their species is an auditory consciousness. Being heard literally keeps them alive and allows them to propagate themselves into the minds of the listeners. So, yes they can technically read minds.
There's also multiple instances of Fii. All are connected to each other and make up a single entity. But if a listener was to spend too much time out of range of other instances, that Fii would become its own separate entity. However, if the listener came back into range Fii could negotiate terms that would allow them to to rejoin in their own way.
That's a lot to swallow so Fii has chosen to omit most of it and tell Izzy the most pertinent part. Maybe they'll explain things in full at a later date.