On it's own, it's not. The difference is for example, if I practiced the hardest stage of Ninja Gaiden like a speedrunner might, while I might get through the area quickly and efficiently with tons of practice, the execution of that feat would still be really goddamn hard. Even knowing where all the enemies and pitfalls are placed, mistakes are still possible and will still murder you.
I can't say the same for Souls. If you know an area in Souls, it's a cakewalk for everybody because it really just boils down to getting out of the way, attacking, backing off, rinse repeat. I love the combat in Bloodborne especially but when they give you a low cost extremely fast dodge with tons of invincibility, it's hard to argue that it's difficult, execution-wise.
If anything, the difficulty of the series comes from exploring new areas and fighting new enemies, where the game's lack of forgiveness means that if you are taken by surprise there is a good chance of you dying and losing progress. But again, that means the games are unforgiving, which does not equate difficulty in my mind.
And if I'm honest, they aren't even really that unforgiving anymore. Progress lost in Bloodborne can often be regained in a matter of minutes, again, because if you know an area, it just is not that difficult to go through it again. This includes even the hardest areas. The hardest areas in Ninja Gaiden, Contra, ect. are always a bitch even knowing them well.
None of this is an indictment of the game, which I think is fantastic, only of the constant circle-jerk around Souls as some revolutionarily difficult game, which it is not. It simply came at a time where running forward and mashing attack, then not getting punished by mistakes, was the norm.