I can't speak for others after all this is my interpretations of what they've written, not what they actually think, but I do not believe both sides indeed consider that exchange to be by the book. In support I'm providing a quote specifically after I provided an
example of play (there were just far too much posts, far too much, for me to include all of them).
This is from another thread on the same topic by the same user but there are others in the same vein in this one, it's just easier to look up a first page of a thread.
So if I interpret their post correctly, there is no exchange, Player A takes a sip and the DM tells the player it's a control undead potion.