and as a parallelism marker

[minshum491]

i saw in one of the manhattan video which explained a OG question.. that "and" can be a parallelism marker only when used without comma like "i like mangoes and oranges" but there is one OG question where there is a comma before "and" and still it is being used as a parallelism marker...can u explain

"There are several ways to build solid walls using just
mud or clay, but the most extensively used method
has been the forming of bricks out of mud or clay,
and, after some preliminary air drying or sun drying.
they are laid in the wall in mud mortar. "

[RonPurewal]

If a parallel structure has two very short components, a comma isn't normally used. If the components are longer, though, a comma can help re-orient the reader's attention.

Much more importantly, the GMAT doesn't test the presence or absence of punctuation, so the presence/absence of a comma is a non-issue in these judgments.