Yeah, dude, you have the completely wrong impression of Sekiro. Sekiro's bossfights are like nothing in any other Souls-like. They are all about focus, concentration, and timing.
Well, according to some people that doesn't mean they are necessarely harder, just different:
Video also mentions you can cheese some bosses with items, so there's some of that too.
All in all i'm not seeing how this game is all that much harder. The lack of stamina bar means you can just spam attacks as you see fit. As someone pointed out to me what this does is that while Soul games encourage patience, Sekiro rewards aggression. Whether this makes Sekiro harder is entirely dependent on what kind of person you are. In Souls you must keep a cool head even when fighting trash mobs. Haste is your number one enemy. In Sekiro, you can actually cheese through some things by spamming some attacks as the video points out.
My hunch is that people have been playing Souls games for so long now they have completely internalized that style of gameplay and don't remember all the times they died due to impatience. They then move to Sekiro having trained themselves to take it slow and are now forced to change gears completely. I think that's what accounts for the sudden jump in difficulty more than anything.
BTW, even though i'm not a fun of jumping ninja shit i'm glad this game retains some of the visual somberness of Souls. I was checking out some DMC5 videos and man i just have an aversion for overly flashy shit. It may be a good game but next to animu crap the visual hypetrophy of a lot of Japanese games irritates me as well.