Otherwise the Klingons would be stuck with the same technology we first saw them with in the 22nd century.
That's what it looks like to me. Other than a minor tweak to something or other which accidentally made them immune to Breen weapons, the energy dampening weapon from "More Tribbles, More Troubles" and the cloak you can shoot through in STVI, nothing has changed between Enterprise in the 2150's and DS9 in the 2370's. They even use the same D7 battlecruisers and Birds of Prey. Their ships are all old rustbuckets, even the "new flagship" Neg'Var from "Way of the Warrior" had the same rusted worn out interior.
I'm all for Ishmael
's backstory, that the Karsid Empire enslaved the tribal Klingons thousands of years ago, and then vanished 600 years ago, leaving the Klingons all their ships and technology.