I’m only on the periphery of the fandom, mostly just watch lore videos and play a few video games. The AdMech is my favorite faction. As for how real the machine cult is, it seems to depend heavily on the writer. My personal head canon is that the machine god is an actual warp entity like Isha and the other Eldar gods before Saanesh nommed them. If emotions and beliefs manifest as warp entities, then the Omnissiah should be one regardless of how the AdMech came to believe in it.