Actually, that was sarcastically quipped by Kira
Twas neither a quip, nor was it intended in sarcasm; actually, it was a statement of incredulity, since as Kira knows, to quote a certain Vulcan philosopher, "Starfleet's mission has always been one of peace."
Sisko only said that the ship was dedicated to Borg-fighting and was "a warship, pure and simple". Doesn't mean Starfleet doesn't build warships as a matter of course
True. It means only that, except for the Defiant, Starfleet doesn't build them anymore
. Every Federation starship since the Constitution class has apparently been dual mission science/military, with no specialized combat vessels still in service. Probably the closest thing they had to such a vessel was the Galaxy's star drive section, and this is undoubtedly the exception that proves the rule, since the Galaxy is a dedicated long range exploration vessel that can't afford to call for backup if something goes wrong.
In any case, it's exceedingly difficult to reconcile Defiant with Starfleet (and to a certain extent, Rodenberry's) vision of what the Federation represents. Then again, it's even harder to reconcile the Borg
with that vision, which is probably why Defiant exists in the first place. Everyone else in the universe there is hope of peaceful coexistence on some level, and even patient cultural exchange to wear them down and win them over to Humanity's point of view. The Borg represent a force of nature that sees no point of view but its own, is unstoppable, unreasonable, and thoroughly impossible to coexist with. Defiant is therefore that last argument of Kings for use against that one enemy that will respect no other reasoning: brute force for a brutal opponent.