You're correct, to my knowledge at least, as far as we know if a mage is killed in the fade they go tranquil.

So either...Felassan is tranquil and Cole just talks about him being 'dead' because he technically is (in the sense of, his personality and who he is as a person is gone) or it was just a writing slip-up.

The other option tho, and a commonly circulated one is just that...Solas is really damn powerful, since he's literally the Dread Wolf, and knows a ton about the Fade. So he did kill Felassan, and somehow actually killed him without making him tranquil, because he has enough control of magic and the fade to do that.

Basically the "He's the fucking Dread Wolf, he does what he wants" assumption.