I completely agree with you; to me a 'good' length ball, all else being equal, is one that hits the top third of off stump. But if you look at what seems to be a pretty standard pace bowler Hawkeye pitchmap, that delivery falls into the 'full' category and the stuff in the 'Good' zone are all short-of-a-length deliveries:
I think it's pretty obvious to all involved that Boult's ball to dismiss Sangakkara did not pitch in that salmon zone; the salmon 'good length' zone defines a good length as not knowing whether to play forward or back, when an actual good length in NZ is something that detonates off peg two thirds of the way up or induces an edge to second slip.
That's why we're seeing the definitional conflict, not because hendrix actually thinks that short-of-a-length is the right length to bowl, but because broadcasters have decided that short-of-a-length is now good and good is now too full.
Incidentally, since the rise of the Salmon Zone so, so many bowlers have ended up dropping way too short for their stock ball. DAMN YOU IAN HEALY!