That's an excellent question and, to my knowledge, there is no documented standard that covers it. I can only offer my opinion.
A boundary is created relative to a potential gas or vapor source, and a physical barrier creates a "boundary" with respect to the source. Conceptually, the desire is for a boundary seal to minimize the migration of vapors and gases through a conduit from one location to another, so it is desirable to seal it where it penetrates a physical barrier. However, where the two locations are at the same relative air pressure, it is not generally necessary to use an explosionproof seal.
Now, if Section 513.8 is what is under discussion, the wall is irrelevant since the slab's surface is what creates the boundary and underground wiring must comply with either 513.8 (A) or (B).