I think you should go back and reread this quote:

Matt said "With regards to the other bosses I've participated in up to this point, this is the only encounter so far where I felt absolutely helpless and at a loss as to what to do. With more time on the other encounters, I'm confident that we would've been able to get those ones down."

In other words, it's not that they couldn't down the boss in one night. It's kind of implied that there were other bosses that he didn't down either. Instead, the grudge against this comes from not really knowing for sure how to approach the fight, even he had more than one night for it.

Not only that, but I'm not sure how you see this as a complaint. Ya, he didn't like the fight out of frustration, but I don't see anywhere that he interprets this to mean that it should be changed or that there's something that's actually wrong with a hard encounter.

Even if there was a complaint about this, lets not forget that Blizzard has aimed to address the difficulty critique by making hard modes rather than simply making all new content significantly more difficult.