I seem to recall that "apostrophes" (at least originally) were editorial by Solesmes. They mean the same thing as a quarter bar, but Solesmes could not insert a quarter bar into the Vatican edition. As for some original composition in square notes by one of our colleagues... it's up to them to explain why they used it!

This mark actually has a distinct meaning in recent Solesmes editions. In the Solesmes Graduale Romanum I believe it essentially was created as a sign that over-ruled a faulty quarter bar in the Vatican Edition. For this reason it is often used with a "tie" over a quarter bar, in other words Solesmes is saying here "please ignore this" and the comma before or after it signifies where the quarter bar should have been placed.

In the most recent Solesmes editions, however, this sign serves a different purpose, and this is that way that it was used in the SEP. This is the description in the introduction of the book:

The final division is called the articulation mark
(,) which is properly not really a division mark at all, but indicates
a need for a very slight break in the sound; a “sliver of daylight”. The
articulation mark is not a breath mark, despite its appearance,
and a breath should never be taken for it. All that is needed is a
very quick and delicate break in the music, as we often break in a
similar way in speech. If the articulation mark presents problems
for beginning singers or choirs it may be better ignored.