Is the Payroll Tax Holiday Illegal?

So asks the WSJ today:

The problem is that the Senate does most of its work by unanimous consent—meaning without objection from present Members and without a vote or quorum. Even a single Senator alone on the floor (or “as a practical matter” one from each party) can use this process to modify the standing order in a heartbeat and conduct business.

The Senate did exactly that to pass Mr. Obama’s payroll tax holiday in December, changing a standing order by unanimous consent to conduct business during an ostensibly pro forma session. Mr. Obama signed that bill. Either that was a real session and therefore his recess appointments are unconstitutional or the bill was invalidly enacted and therefore unconstitutional. Both can’t be true.

That seems correct to me–that either a pro forma session is a real session or it is a recess.  It cannot simultaneously be both, can it?