It's worth noting that other early games did not use XP.
1975 Tunnels and Trolls uses either AP (Adventure Points) or EP, depending upon edition.
Runequest (1976) and Traveller (1977) didn't use experience points at all.
1974 & 1976 † Original Edition D&D doesn't use XP nor EP. Electrum are mentioned, but non-standard, and Experience is spelled out.
1978 Starships and Spacemen, by FGU, abbreviates Experience Points to Exp. Pts.
1978 Holmes Basic D&D (Blue cover) spells out Experience, but does use EP for Electrum Pieces.
1978 AD&D Player's Handbook uses E.P. on page 20 (in the Cleric description) but nowhere else, and spells out experience everywhere else it's used.
1979 AD&D Dungeon Master's Guide uses X.P. and XP (many pages, defined on p. 230), and defines EP and E.P. as Electrum Piece (p. 228)
1981 Mechanoid Invasion (Palladium) uses Experience, spelled out.
As noted, 1981 Moldvay Basic D&D uses XP and EP.
Therefore, Moldvay is not the origination of the use.
Moldvay also was not in print as long (it was replaced in 1983) where AD&D was in print with literally only cosmetic changes until 1988‡ or so.
Moldvay was quite popular, while it lasted, but had less lasting impact.
Therefore, it is most likely that AD&D is the popular source, not Moldvay's Basic D&D. Further, looking at many early videogames using XP, like Final Fantasy, the classes are closer to AD&D; the inclusion of Rangers, Assassins, or Druids, and separation of Race and Class are classic features of AD&D that were absent in Moldvay.
† 1976 revised some wordings and renamed several monsters due to a lawsuit from Tolkien Enterprises.
‡ AD&D 2 was announced, and AD&D 1 was no longer in print. 1E would be found in stores for several years on, as stocks in distribution continued to sell.