Yes, Biden was trying to argue it was within the law.Well, yes. Spending money that wasn't authorized to be spent is different than not spending money that was authorized to be spent.
Keep in mind that the "not spending" parts that we're seeing at this point in time are pauses, or freezes. Laws do often empower the executive branch to do that -- which is distinct from wholesale impoundment (which is permanent).
While POTUS used to have wide latitude for impoundment (and I think it would be great if they got it back....whatever the party of the Congress or president), Congress limited it by statute. That particular law has never survived judicial review. And I'm sure it will come up in the courts as they review all this.
But even the Biden Administration didn't argue that they could spend money that Congress never authorized. They argued Congress did authorize it in the HEROES Act. The argument just didn't prevail in court.
Trump doesn't even care what is legal or not. Just do and dare someone to stop him.