Thats not how it works the animations aren't client sided anyways and if youll remember unblockable worked by having you throw a side attack but even on the client side screen it was still a normal attack. Also the animation cancelling/glitch shows up on both sides so you can't block without it showing up on the other person's screen.
Animations are client side. That's why the NW enhancement mod and the like are able to work. Your animations do not have to line up with the server's. The server will send your client a packet telling it that there is an animation in progress and it's up to your internet and client to process that packet.
There is a problem in Warband with animation desync. This has been the case for a long time. A lot of people over the years have experienced standing there holding an up or down block, then being told that everybody else in the server saw them holding a sideblock. I get the same bug with attack directions, mostly when I'm playing cav. I believe it has to do with packet loss and changing attack/block directions quickly, but it's purely conjecture. I get it most often on cav, when I'm flinging my mouse around to look for enemies and then quickly try to grab an attack direction.
Taleworlds even is/was aware of the problem. There is a built in delay between when your client registers you have clicked and when the packet gets sent to the server. They increased it in the January 2018 patch. That was their attempt to resolve it. It didn't work, and really didn't accomplish anything other than making melee feel garbage and getting people to shit on the actually positive melee changes that FSE made. You can see this discussion about the change
here. Keep in mind that, although FSE reverted all of the changes on their end, the change in question was a Taleworlds thing in the Warband .exe, and to my knowledge, was not fixed.
Without making this a rant on how Taleworlds ruined NW, wrong blocks have been appearing in the game for a very long time. The fact that you're seeing it is more likely proof of Warband's awful networking than it is of cheating, unless you're seeing it reliably and consistently.