I don't think this is the case. Other M is part of the main continuity, it just happens way before the Prime games do.
Other M is the prequel to Fusion. Everything that we see in other M is something important to fusion in some way.
Metroids bred as bioweapons, Ridley revived fully and drained by the Metroid Queen, and subsequently transferred to the BSL Station's cryo sector, the original nightmare in his initial debut, and also subsequently has his remains moved to the BSL, and reconstructed,
And then theres Samus and Adam, the whole underlying story behind Fusion and Samus' internal monologues about him. this mysterious Adam whichy we never knew much about, only that he sacrificed himself to save Samus' life, and that his mind backup was instated as her Ship's AI.
The primes take place right after Metroid 1/Zero Mission, since Metroids still exist.
it goes:
Metroid Manga -> Metroid -> Prime -> Prime: Hunters -> Prime2 -> Prime 3 -> Metroid 2 -> Super Metroid -> Other M ->Fusion
it was a NoA guy i think that said they were a "separate" collection of games with a different concept and style or some thing.