WMG / Star Trek Deep Space Nine S 07 E 07 Once More Unto The Breach

The Klingon D5 class ship mentioned in this episode is not the same D5 that is later shown in Enterprise

This is a reuse of an old classification. In the Star Fleet Battles gaming universe, the D5 class is actually a variant on the D6 class (D7's immediate and almost identical predecessor). D5 is a "war cruiser" variant using D6 surplus hulls which are still being manufactured cheaply by at least one shipyard. The D5 is relegated to light cruiser, troop carrier, testbed for new technologies, and anything else so that the more up to date D7s can focus on their primary mission. Oh yes, some D6s are also sold to the Romulans, who in this gaming universe, are in dire need of more up to date ships so that someone will take them seriously. So no, Kor wasn't being senile and he was correctly referring to the ship we saw in The TAS episode Time Trap. And the D5 designation is just a reuse of an old classification for a discontinued production line. The D5 seen in Enterprise is part of an old series of Klingon cruisers. The cruiser series is rebooted to D1 at some point after the Romulan War and that is where Klingon ships adopt their more streamlined look.