Thanks for playing!
Yeah, I kind of expected that most people who'll play this game when it releases would have played Sekiro/similar titles before or have at least seen gameplay of those games, so I didn't even think to explain those... Actually, he can still be beaten normally, just that the finisher will only kill him after you finish the tutorial section. This part is really badly implemented, since you can do the finisher before that so it's probably really confusing.
For blocking/deflecting, the block starts the instant you press it, but parrying requires you to time your attacks with the enemy's, so there's an inherent delay in when that comes out. I guess I could try to avoid that issue of starting attacks after the player by increasing the startup for combo-starting attacks? Currently, players and bosses are animated at pretty close frame numbers, with the player being slightly faster in most cases, so that's probably why it feels that way.
Since the attack hitboxes are tied quite closely to the weapon itself, it's possible that you just managed to get out of the way while blocking?