Hello,
The point of the matter here is that the MUL is a living document, constantly reviewing and verifying the legality of the various units fed into it. A unit that does not appear which may have been referenced in an official source reflects one that has either not been added YET, or which has proven illegal for some reason (bad construction rules, faulty premise on the part of the writer, in-universe retcon, and so forth).
So "not on the MUL = not in canon" means simply that the unit's canonicity is either pending or has been purged as a bad idea. If you want it on your table, nobody's stopping you, but don't expect us to support fiction and/or source info based on a unit model that has not yet been properly vetted and verified.
Thank you,
- Herbert Beas
BattleTech
Catalyst Game Labs