Ive gotten multiple legendaries over a span of 20 minutes or so its definatly not the case.
Anyways here is their justification on the monster HP pools
"It took us some time to evaluate whether or not we wanted to alter some of the current most popular runs. Our baseline for comparison for these alternative runs is to see how they stack up versus Adventure Mode and Bounty runs, and these different types of play should be on par (or at least fairly close). After extensive testing, we were finding this wasn't the case and came to the decision to make these changes. It's a decision we wrestled with for a while, but ultimately, we want to avoid situations where the most efficient game play isn't necessarily the most engaging and our investigations revealed this to be that kind of situation. We want a larger variety, rather than only one or two particular routes, to be the most rewarding way to play."
And Sets
"We've been listening to player feedback on difficulty rewards, and one of the most frequent comments we receive is that Torment difficulty isn't rewarding enough. As a result, we've reached the decision to make class-specific sets, specifically Natalya's, Immortal Kings, Tal Rasha's, Inna's, and Zunimassa's, drop only in Torment I or above from this point forward.
The class sets weren't designed as a progression, but rather as a lateral choice between different play styles. It makes sense for all of them to be obtained at the same difficulty level, alongside the other sets that will be available at 70 in Torment I+. This should also make these very strong items a little bit more rare, and when you reach level 70 and you see someone wearing a full set, you'll know the level of play they needed to reach in order to acquire those items.
The reward level of each difficulty setting and how they feel is very important to us, and it's something we'll continue to observe and tweak as we enter the expansion."
I am guessing this means CoTA and T4 Cath runs took it in the ass, but I did neither so I don't know what they meant by mobs with lower then average health pools.