Let's remember that the Cardassians made their actual TNG debut in a manner that suggests they could have been known all along in TOS and ENT, too. We were simply informed all of a sudden that these never before seen or mentioned scaly villains had been Federation arch-enemies for the better part of the century already.
Ditto with Klingons in TOS, come to think of it. Perhaps our heroes don't like to discuss folks they have nothing good to say about?
As for the Cardassian poet, Iloja of Prim, Tobin Dax met him on Vulcan. Doesn't tell us much about the timeframe, but it would fit the general pattern of Vulcans hosting exiles and oddballs and allowing them to keep their secrets (as the Trill secret wasn't leaked out to the Federation, either). It could well be that no human ever heard of Iloja or his species from the Vulcans.
Timo Saloniemi