I guess it depends on the nature of the "founder-Kahless". The factoids we are given establish him as a famed warrior from a couple of thousand years back, and we know that such people in Earth history are shrouded in myth when comparable timespans are involved (King Arthur, say). But if Kahless was also the ruler of a fairly well organized empire, it's highly unlikely that the Unforgettable could have been forgotten: history books on him ought to be
b) written back in his heyday already, by people in the know
c) not rewritten by his enemies, as his heritage appears victorious and unchallenged
d) hence, pretty reliable.
We know rather exactly who Gaius Iulius Caesar was, despite there having been dozens of leaders of that name around the time, each contributing chapters to Roman military and political leadership history and even to the succession of emperors (heck, the first emperor, Octavianus "Augustus", was named Gaius Iulius Caesar for a while, too!). There's no ambiguity about the "one" Gaius Iulius Caesar. And Kahless is bigger than that.
Sure, Kahless is also half Jesus Christ, but nobody wrote about Jesus Christ when he lived and died because back then he was considered a nobody. Kahless was already famous in his life (and, supposedly, in his death, although we haven't heard the story of that yet). So there wouldn't be any "gospel" of Kahless, composed ex post facto
, just dull factual writings with the mythology scabbed onto that and, if necessary, easily peeled away again.
There's even the scenario in which the Klingons were starfaring already during Kahless' reign. Whether with indigenous ingenuity or using hardware taken from their Hur'Q slavers, this would provide them with advanced means of records-keeping as well, making Kahless all the more historical.
I guess that one is the most interesting scenario: Kahless comes from such a primitive setting that he can be credited with forging the first sword, but comes to evict interstellar overlords and give his people the stars... But something more mundane is probably closer to the truth.