Yeah, pretty much. The tengu is the one to name you Sekiro and Isshin immediately knows you by that name. And if you miss the Tengu by the Serpent Shrine, Isshin will instead ask you about learning techniques in his chambers later.

After completing the game I’d say it’s highly unlikely that they are one in the same. There’s pretty concrete evidence

In isshin room, behind him, its the cloak and mask...

What exactly suggests that they're two different persons? I think it's blatantly obvious Tengu is Isshin.

I haven't completed the Ashina Arts, but I did get Floating Passage from the Carp Merchant and Tengu still gave me the Mushin Esoteric Text. It could also be because I have the Shinobi Arts completed maybe?

you only have to unlock the last skill from any tree

Spoilers like this wouldn't be in a note in different games' wikis, they'd be in the introductory paragraph. Just because the game is new does not mean that there should be spoiler warnings in the wiki. You shouldn't be in a game's wiki on your first playthrough if you don't want spoilers, it kind of comes with the territory.

Massive spoiler? It's implied they're the same person rather overtly in the conversation with Ashina right after the Genshiro fight pretty early on.

He literally refers to you as Sekiro after Tengu is the only person that calls you that...The uniform is on the ground behind him...It's blatantly obvious.

After th attack he can be found outside the upper tower dojo window.

The abdandoned dungeon or just anywhere you can get into Ashina Castle except from the main gate.

Go to Old Grave idol and make a 180 and take the next right. The bridge that was broken is now fixed and just proceed until you make it to an idol that teleports you to DoH bossfight.

I did it from the demon bell. In the room with the headless there is a path that gets you to the building exactly where you first fought there chained ogre

Defeat the demon to access the building.

You mean SOGEKINGU?