Can Chair unilaterally adjourn meeting?

Newly-elected Board Chair James Shadd has adopted a change when arriving at the last item on an agenda. When it's time for a motion to adjourn, he has begun stating something to the effect of, "Seeing no opposition, we will adjourn."

While Shadd is Chair, he really is just one member of the board.

He doesn't "see" any opposition to adjourning, because he hasn't called for any opposition.

Does this follow the Robert's Rules of Order? It's past time for Attorney McFadden to come back to the Richland 2 school board for an additional training session regarding Robert's Rules.

Should he respect the other members of the board and ask for a Motion to Adjourn? One member would make the Motion; another would second it. He could ask for a hand vote. (Voice votes should not be requested in Zoom meetings.) That would take about 15 seconds.

In effect, he muzzles any board member who might wish to speak.

Why has he changed a long-standing procedure?