Unusual exceptions to standard norms are just that - unusual exceptions. There's nothing wrong with having something that is distinctly not the norm, and indeed, if we never encountered such things, the world would be a much more boring, drab and mundane place. As such, an individual example of something that stands starkly against the normative established lore, and is clearly noted out and lamp-shaded as being so, is perfectly fine; it is its unusual nature that is interesting.
In the case of the Ormellum, he is an illithid arcanist; illithids have a strong distaste, almost disgust, for those with arcane abilities that occur with their own; one of the less stated reasons for this, however, is that these unusual individuals usually have a more pronounced sense of self and are far more likely to find themselves at odds with the will of the elder brain. Those that do find themselves at odds with the will of the elder brain, or craving more free will of their own are either executed, or else they escape and break free, going to ground and trying to make sure they don't come back into the influence of their, or any other, elder brain ever again. they become rogue, independent illithids. This is very rare, but not unprecedented.
As such, finding this rogue illithid down here, hiding and doing research as he looks for a 'better way' to be and to live, is extremely unusual and a very unlikely thing, but it is also entirely believable within the realm space.