That's not an indication that the decision was made without HBO's input, but the opposite. How else would Benioff know that they would've liked more episodes and that they understood why it should it end where it did? They no doubt had meetings in which D&D made their case for a shorter season and HBO agreed to it because they were convinced by their argument and trusted them. Don't confuse HBO agreeing with and trusting them with having no input and no choice but to do whatever D&D wanted.
How do you figure?
Here's a possible series of conversations:
D1: Hey, you want to end this series early?
D2: Yeah, man. Let's do it.
_____________________________
D&D: Hey HBO, we're going to end the series with two short seasons, K?
HBO: Well, we'd like more episodes because this series is an amazing cash cow for us, but this is your show and meddling in our showrunners' work could have adverse affects on our future business, okay go for it.
D&D: Yay.
Seems pretty straightforward to me. Given quotes and information we know, that is far more likely than HBO being behind shortening their most successful series in history. It just makes absolutely zero sense for HBO to want to end the show early and haphazardly, because that could hurt their brand a shitload - which it has.
This is the series of who is at fault (top being most at fault):
1)D&D
2)GRRM
way down here...
111) A beetle.
172) HBO for getting in bed with D&D and not having the premonition to put a non-compete clause in their contract forbidding them from signing onto Star Wars before finishing GOT.
He did, but permission to adapt the story isn't the same as the rights to it.
Indeed, which is why the lack of information is the problem. For all we know there could be a contract somewhere that states HBO has the rights to the series if and only if D&D are the showrunners. No clue without knowing everything about the deals they made more than a decade ago.