my frlg.ips theory (ABYSS ROUTE SPOILERS!)
further spoilers below
It also works because OMEGA and ALPHA were the only ones that "were"; i.e., they could be encountered in the original Red and Blue (via glitches), but were not truly "of the LAND" because neither could be obtained via normal means. That's true of Mew in RB because it was event-exclusive, while it in Gen 3 it was exclusive to Emerald, and thus not part of FRLG, i.e. the LAND.
As well, if you do the normal route of FRLG.ips and catch Deoxys, you get the Old Sea Map that would take you to Faraway Island where Mew is, which is... not a location in FireRed or LeafGreen, obviously, but will get you a special interaction where the Sailor reveals that Mew was under the truck—another classic Pokémon Gen 1 hoax, which naturally fits it in as part of the ABYSS's domain. The Sailor even describes Mew as an anomaly.
Lastly, Mew is, well, the last index in the RB Pokédex, whereas MissingNo. is usually programmed as the first, or as index #000. MissingNo. is the first, the ALPHA, while Mew is the last, the OMEGA.
