I still have my official The Motion Picture blueprint set. According to the Starfleet Intelligence data attached to K't'inga, they were newer designs than the D7 - not refits - and were first constructed sometime previous to TMP (2270's). The armor along the hull was in addition to what the old D-7 carried. They typically patrolled in packs of three. A Refit Constitution - which wasn't a reality until later - was expected to even the odds should it encounter such a pack. The set implied that the K't'inga were nothing to sneeze at. Yet the refit Constitution was expected to be able to take on such a pack all by itself. Though no film or computer game I've experienced ever tried to put that interpretation into practice. :p
Looking at the blueprint set now, I speculate that the K't'inga had ablative armor before Ds9 introduced the buzzword via Defiant. When STO gets around to a retrofit, I'm hopeful for the skin art to be completely redone. The movie model displays placement of the armor plating like a work of engineering art. Ours should look as nice.