Would this ruling make Nixon’s actions in Watergate legal too?
It would make it illegal to use the tapes as evidence against him. So it doesn't matter if it makes it legal or not, because it makes the illegality impossible to prove in a court of law by denying evidence to the prosecution.
People are jumping on language in the decision that says discussions or probings about the criminal nature of the crime would not be admissible.
So Nixon ordering Watergate would still be admissible - Nixon discussing with his legal team or cabinet after Watergate broke would not be.