The bull is simply another example of a boss that's out of place with Sekiro's play style. The Guardian Ape is an entirely different fight that rewards you for being aggressive. The Demon of Hatred isn't even that difficult, it's just tedious.
I don't agree. If the bull was alone, it might be, but the fact that you have all of these non-human fights and how they introduce and evolve your playstyle against them is quite fitting for the game, imo. Moreso that DoH is an optional boss, it doesn't need to adhere as much to the core systems as, say Genichiro or Guardian Ape would. Sure, they could have made the fight shorter, but that's another debate entirely. You're saying it's "out of place" when it's not really, considering you can fight both the bulls and the guardian ape fights before this, and Demon of Hatred uses some of the same characteristics of those fights while building upon them in a unique way.
If there were no other non-human bosses in the game and DoH came out of the blue, I might see this point of view, but with the way the bosses are designed that I mentioned earlier, no, I simply don't agree.
Edit: This isn't even taking into consideration the lore and story connections the boss has. The game has been building up for the reveal of this boss since the beginning of the game, and has hinted and teased it throughout. It is a well thought-out addition to the game, and ties directly to the themes and story in Sekiro. It's probably not what you meant, but these comments make it seem like they just lifted a boss from Souls or Bloodborne and dropped it in. But it's simply not true; it's tuned to Sekiro's combat and mechanics, fits as an extension of other non-human fights in the game, and ties heavily with the story, lore, and themes of the game.