Why not? They are already in the game and as far as we know, not significantly older than the B'rel.
Unlike the Federation, the Klingons seem to stick with tried and true designs. They came out with what, one new ship design between TOS and TNG?
Because this game supposed to be the future. I think they explained that plenty of times regarding people wanting older versions for T5 ships. :p
And as for new KDF ships between TOS and TNG, we honestly can't say. If there was ever a new Trek that fills in the gap, that could easily change. But I'm guessing your argument is that they had basically the K'Tinga, yet there was the D-12 BoP, the B'Rel, and the K'Vorts that also was created during that time frame.