This is really not supposed to be a knock on you, WisconsinCard, but I do not believe what you have been told. If it was in fact true that every team knows what other teams will do, there would be no reason for all the secrets and smokescreens each team uses a lot of time and ressources to put out in the months leading up to the draft. Each team could care less if the fans and the media know their plans if all the other teams know them as well.
I obviously do not know if the Cardinals actually did want to select Watson with their pick or if all the talk of how they wanted a quarterback was only various smokescreens, but I do know that two teams paid a lot to move ahead of the Cardinals. Furthermore, I can imagine that in order to trick those teams into doing so they might have told Mahomes and Watson that they wanted to draft them so that both of them could tell that in meetings with the other teams.