18 years is not the same as the same year.
It used to be, back then...
But then technology changed, and so did our ideas on the passage of time.
had Starfleet's guard been properly up, I can't imagine them being gullible enough to fall for the metagenic weapon ruse
Hmm... I for one think that the very alertness of Starfleet is what made them misjudge so badly. Had there been complacency or self-assurance rather than this terrifying certainty that the Cardassians were up to something, a more objective reaction to the metagenics disinformation might have followed.