Well, it's not primarily about prospects, I don't think, but more about what we characterize as AAAA guys.
In a normal season, if a prospect is on the 40-man, but optioned, he can be recalled to the active roster and re-optioned repeatedly - but he's not typically removed from the 40-man. This summer, a prospect would be treated the same way, in general. If he's included in the 60-man pool, he can be added to the active roster, and returned to the inactive pool, repeatedly.
The difference is that, in addition to the fifteen-odd prospects that are on the roster, but on optional assignment, and can thus be rotated on and off the active roster, clubs have an extra 20 players - guys who are not on the 40-man roster - in their player pool. These are equivalent to the AAAA players that organizations stash in AAA in a normal season (it's essentially the whole AAA roster!)... and in a normal season, when these guys are brought up and sent back, they have to go through waivers. That doesn't really appear to be very different for 2020.
The wrinkle this season is that, with the AAAA players, a club can move them on and off the active roster freely, as long as they stay in the player pool. If they want to get the guy out of the player pool altogether... well, then that's a one-way decision. If the player is of little value, he might clear waivers, and remain club property... but would have to go home for the balance of the season. If he's claimed, he can be added to another club's pool.
But the more important point is that the player pool means clubs have increased flexibility with regard to these AAAA type guys. And as far as the real prospects on the 40-man roster (for the Phils, that's Deivi Grullon, and some pitchers - Medina, Romero, Cleavinger, E. Garcia, Llovera, maybe Sanchez), they can just stay in the player pool - if they're included on Sunday. I'm assuming that Gamboa will not be in the player pool, and some of the pitchers (Cleavinger? Llovera?) may not, either. In addition to the "normal" active 26 guys, the Phils' pool is likely to consist of about five real prospects currently on optional assignment, about nine marginal players on optional assignment, and about 20 eminently replaceable AAAA types.
If there's an "intent" here, it may be to discourage clubs from including young prospects, who are on the 40-man for Rule 5 reasons, in the player pool - because they don't belong there. Arquimedes Gamboa. If a club only includes the players on optional assignment who actually have a chance to help the club... well, generally that's only going to be a handful of players in each organization. The rest of the pool is the "deep reserve" of AAA players who typically get selected/ outrighted/ selected/ outrighted each season, but who can be moved in and out of the active roster repeatedly under the special rules. If a club wants to dump one of these guy from the pool completely, to make room for another such player, it's completely appropriate to require waivers; in a normal year, that player would be exposed to waivers every time he left the 25-man roster.