...just because I do not agree with someone's explanation of the mechanics...
Did you try pressing down on the barrel/chamber with the gun in lockup?
That should satisfy you that the barrel and locking block are not usually in contact when the gun is in battery. In fact, the small lip on the bottom of the barrel is held against the slide lock by recoil spring pressure. From that point it should be easy to understand why the smaller groove in the MGSL puts the barrel lip (and therefore the barrel) in a more consistent position compared to the larger groove in the standard slide lock.