Not that he needed to. "High crimes and misdemeanors" requires no breaking of a specific law, just a failure to keep the public trust and perform the duties of office. But the Impoundment Control Act does add some legal force to the overall case of malfeasance. And it specifically underscores that the ICA "does not permit the President to substitute his own policy priorities for those that Congress has enacted into law."
And there's still obstruction, which I think the Democrats need to work a little harder at explaining and clarifying. I'm still seeing GOP Senators and others in front of a camera saying "the House should have finished its job, so that no further witnesses are needed. Lazy old House, nah nah nah." It is important to clarify why this is incorrect - that the WH refused subpoenas, coerced potential witnesses, and blocked legal document releases which hobbled the House committees in the completion of their duties. More witnesses are needed in any case where highly relevant new material comes to light and new witnesses are available - especially, when the Senate portion is, in fact, the actual trial.