It isn’t daft. The Republicans since Reagan have pushed a fringe legal theory called the Unitary Executive Theory. Basically, they want the president to fully control the executive branch and military such that theirs is the only voice that matters for much of the government. Not unlike a king, but partially checked by congress and the courts. They have been taking (illegal) actions to try to get sued, and also have been suing others/other branches of government, to try to get the Supreme Court to hear cases that will support this fringe legal theory so that it becomes the law of the land.

I am not a lawyer, but this is possibly something Trump can legally do since he is Commander in Chief of the armed forces. However, this seems more like an apportionment thing, which is Congress’ responsibility. Congress has allocated funds to send military aid to Ukraine. So, even if Trump as Commander in Chief could say “no more weapons to ukraine”, it seems doubtful to me that he could (legally) stop weapons shipments currently en route.

But, by the time whatever government office sues the office of the president to get a judge to enjoin them to send the agreed upon weapons that were already apportioned, it will already have hurt Ukraine somewhat. Trump often weaponizes inefficiency. And these sort of illegal acts aren’t crimes per se - they’re just procedural breaches - the legal remedy is just to reverse the action.

So, probably not legal. But Trump gets to weaponize his administration’s incompetence (or feigned incompetence) to at least delay aid. More competent people may support these actions, knowing they’re illegal, to try and strengthen the president’s role even further.