I just went and looked: the exact wording regarding Mordock is "first Benzite to attend Starfleet Academy".
Umm, the usually reliable "Chrissie's Transcripts" at chakoteya.net have Chang saying
Chang: "Congratulations, Mister Mordock. You're the first Benzite in Starfleet."
While Chang does seem to mean that Mordock is the first Benzite to end up in the Academy, the wording as above doesn't support the idea that some other Benzite could have ended up in Starfleet by other means. And in any case, the contradiction here was that Sisko and Laporin both went to the Academy - so if Mordock was the first to attend the Academy, the contradiction stands even if other Benzites entered Starfleet by other means.
So we still need an explanation, which can come in three varieties:
1) Mordock wasn't literally the first Benzite ever in Starfleet Academy, despite Chang's turn of phrase
2) Laporin wasn't exactly studying in the Academy, despite Sisko's turn of phrase
3) Benzites and Benzenites are not the same thing
On the latter matter, let's remember that it's a Klingon calling Laporin "Benzenite", not one of our heroes. The Klingon could get the nuances wrong... Much like some humans' "Klingots".
On the second point, though, Sisko and Laporin
could have gone to the Academy 8 years prior to the episode, not 22 years prior. Sisko no doubt had to take additional training when abandoning his engineering career and going for the XO job as encouraged by Captain Leyton.
Timo Saloniemi