If the argument were about letting the courts decide on executive privilege, that would hold more weight, but that's not what's happening. Trump is directing people to not even show up and is refusing to turn over any documents. If he were claiming executive privilege, witnesses would show up to testify but wouldn't be able to talk about certain topics, and documents would be turned over with certain items redacted.

The "expedited" McGahn case was filed August 7th and still has a couple rounds of appeals left after the current round is ruled on. And that case is just for him to show up to Congress to testify, not on any claims of executive privilege, which would spark another, new court case and months of appeals. Even if that case is also expedited, we're looking at well over a year for everything around McGahn's testimony to get resolved.

We also now have Bolton having come forward with new information and saying he would comply with a subpoena by the Senate. And, as hoojinks mentioned, executive privilege certainly doesn't apply to information that's about to be published in a book for the whole world to read, so no court case would even been needed to get Bolton's testimony right away.