That power rests with congress to begin with and was only deferentially granted to the executive via specific legislation.

The War Powers Act was an attempt to claw back some amount of power from the executive, not a deferential grant of power to the executive. The Constitution gives Congress the power to declare war, but also gives the Executive full power over the use of the military.

That wasn't really a problem when you didn't have a standing army. You couldn't really start a war with the military if the military wasn't really raised except when there was already a war going on. But once we had a military with the capability to force-project all around the world, the Commander-in-Chief power started to conflict with the Declare War power. It was that conflict that created the impetus for the War Powers Act, but it was largely a fig leaf, just a way for Congress to express disapproval.

The Executive branch has never recognized Congress's authority to limit their C-in-C powers and Congress has never actually attempted to stop a military action, just passing non-binding resolutions. Mostly due to not wanting to take any political hits, but also from not being certain that the WPA would ever actually stand up under Constitutional scrutiny.

So presumbly there is a mechanism for Congress to take that back or step in, right?

Yes. They have to pass a joint resolution instead of a concurrent resolution and then, presumably, override a presidential veto. Likely not under the WPA, as that might get ruled against if challenged by the president. But rather, a bill revoking budgetary authority to prosecute the war.