Hasn’t this already been decided on during the Nixon Admin?

Yes. Nixon had a much stronger case than this. Nixon argued that the audio recordings he had made of his meetings, that he'd always intended to keep solely for his private records, were his personal property and protected by executive priviledge. The Supreme Court unanimously ruled against him.

Trump's trying to claim executive privilege over the work product of a team inside a department several levels below the office of the president. It doesn't remotely resemble private advice to and deliberations of the president. There's no way courts uphold this without exposing judges who actually want Trump to be above the law.

It's frivolous in that it's a ridiculous claim, but it's very serious in that if Trump wins anyway, checks and balances are over.