Why Trump's threat to adjourn Congress is dubious

Are the House and Senate in disagreement?

No, so Mr. Trump would not seem to have this power available to him.

After passing the $2 trillion pandemic relief and stimulus package on March 25, and just before its members scattered, the Senate majority leader, Mitch McConnell, Republican of Kentucky, proposed going into three-day pro forma sessions. The Senate approved by unanimous consent.

The Senate did not ask the House to adjourn for a longer period, so there has been no opportunity for them to disagree.