Taiga: As in the works of TYPE-MOON, even where the works are different, world settings are often shared, would it be true to state that outside of the world of EXTRA, the Moon Cell does not exist?
Sakura: Indeed, it does not exist. This would also be the reason that Fate/EXTRA is referred to as a World of 'if.' In the first place, were the Moon Cell to exist within it, Tsukihime -- which shares the same world settings -- would from its very premise collapse.
Saber: Mu? But I'm of the impression that Tsukihime characters appeared within the primary material of the game. Was that a hallucination?
Sakura: It wasn't a hallucination, but -- the matter is that there is no direct continuity of lineage from the History of Man that exists as of Tsukihime. To put it bluntly, it was fanservice.

Fate/EXTRA Sound Drama 2 Bonus Drama