I think it is a ProSeries thing, not an IRS thing.

Up until this year, there was no way to designate an e-filed 1040 as "superseded", and the IRS won't allow duplicate submissions, so the 1040X was a way to tell the IRS it was a superseding return.

But now that 1040s have a way to indicate they are superseding return, a 1040X shouldn't be necessary.