In my opinion exactly this is what happens, when player who have monsters learned try to metagame using that knowledge ... but GM is homebrewing (or just slightly alterning, to not be so dramatic) things to make that impossible to them.
In other words, i believe situations like this are exactly the reaspon Rule Zero was created for.
The thing is though, why would you change this specific detail the way Larian has? They've buffed him in numerous ways and took away numerous weaknesses already, yet have given him this weakness that makes him more of a generic vampire, and taken away an aspect that's specific to the lore. There's a lot of stuff that's explained by the tadpole, but not this. In this case of the stake, they just actually took away something interesting and unique about the setting.
