Treated as such by whom? Certainly not the production teams who didn't stick with a consistent design approach from one movie to the next

Memory Alpha and Ex Astris Scientia both treated the Klingons as consistent from TMP through ENT. It wasn't until the deleted scenes from Into Darkness came out that the design was actually changed, and even then if they hadn't changed the eyes one could've just assumed they were Blinged up versions of the TMP iteration.

In season one L'Rell and other Klingon women had elongated skulls that are no longer present in season two. There are other changes as well beyond their growing hair, but that's the major one

There's a few background klingons with the elongated heads. This is a case where the stated change doesn't line up with the production change.

Point is, small variations to the same design ≠ total redesigns, which have happened only twice.