When Jeremy says, "you're a Betazoid, don't you know what I'm thinking" or something to that effect, he is assuming that she is a full empath.
He says she knows what he's feeling
, which was the question she asked. And that is well within her range of abilities. As the scene itself made clear, she didn't ask about his feelings because she couldn't sense them; she asked because she wanted him to talk about them, to face them and work through them as part of his therapy.
However, is it true that some Betazoids don't have the ability to sense emotions at all? I think Tam Elbrum mentions that in Tin Man. Again, a source of prejudice to assume all Betazoids are fully empathic.
Most Betazoids are full tele
paths, able to sense others' thoughts as well as emotions, though some are more sensitive than others. Since Deanna is half-Betazoid, she is merely an em
path, i.e. able to sense emotion but not thought -- although she is able to communicate telepathically with people she's particularly close to, such as her mother and Riker.
There are probably other Betazoids who have even weaker telepathy or none at all, just like there are humans who lack sight or hearing or the ability to recognize faces. But Deanna wasn't some random Betazoid that Jeremy knew nothing about. As a member of the Enterprise
's complement, Jeremy presumably knew who the ship's senior officers were and what they were capable of. He wasn't making a guess about her species; he knew what abilities she personally had. But again, it's beside the point because she wasn't asking for her benefit, but for his.