They actually did have Fusions in Ent. And maybe the Fusion process limits lifespan?
No, they had Augments with the human appearance being a side effect. Fusions were deliberately bred for infiltration work and to change the Klingon mindset to be better able to understand the races they were fused with.
Subtle but important difference.
I like how TFR reflects the Klingons of their day in parallel with the Soviet Union analogy that was the basis for the TOS portrayal of them.
The continuum starts with Imperial Russia (Enterprise-era), goes to rigid Soviet-style statism (TOS) then back to something more like pre-Soviet Russia (sometime before ST VI).