A one card straight flush on 5
6
8
9
is just as combinatorically likely as a one card nut flush on any four heart board (well, perhaps slightly less likely as people play Ax hands more frequently than 7x). There's nothing "ridiculous" or especially unusual about it when you're playing against people who know the hand rankings and how to read the board, other than the absolute hand strength being on the high side. In this hand your K high flush is essentially the same as a Q high flush and you should play it as such, which would seem to entail vbetting against this villain.