He's mentioned those ships once before. I had to search Memory Alpha to figure out who they even are (ancient Klingon enemy). My guess is he's giving hints about the "unique Klingon enemy" that's included in their storyline.
It's possible. The Fek'Lhri were around during the time of Kahless the Unforgettable and he used the first Bat'leth ever forged to conquer them, as mentioned in the DS9 episode The Sword of Kahless
The Fek'lhri were "demons" We once saw one that was supposed to be the guardian of Gre'thor, kinda like Cerberus in the greek mythology. If they want to give us a realistic enemy from the Klingon past they should take the Hurq.