I loved the explanation. I thought it very satisfying and well done as well as being an interesting story in itself.
I agree. I'm not usually fan of fanwanky unnecessary explanations (like the nuTrek comics trying to explain why Khan is British now or the ENT novel that tried to explain why TOS looks less advanced than the NX-01), but this one was harmless fun and the added backstory didn't negatively taint any of the old Klingon episodes.
Plus, when I first saw it i had no idea they were going to it, and I'd somehow missed the teaser where we see a Klingon's forehead dissolve. After spending three years saying "these pre-TOS Klingons shouldn't have bumpy heads!" (albeit jokingly), I actually stood up when the Enterprise was boarded by a squad of TOS-style human-looking Klingons (not unlike Sulu at the start of TUC
). A huge OMG WTF moment.