Well, as far as he knew, Alphamon was neigh nonexistent, Dorugoramon was like, a .00000000001 probability, and Millenniummon is a nearly impossible anomaly, or something he didn't think anyone had the means of even researching.
Remind me because I forgot: Did Jordan not know that Alphamon is the first Biomerge result between Doru and Zei? I'm sure he'd seen her as Rodendramon (correct me if I'm wrong) kicking ass against the Knights, but I'm not sure if he's aware of that fact.
As for Dorugoramon, I agree that nobody (save for only one of Ob's character) knows about her existence at all, not even Doru and Zei, so that's okay. People familiar with tropes, she is a Giant Space Flea from Nowhere as far as our characters are concerned (Psyk gave me the a-OK).
For Millenniummon, you gotta ask Ob and Psyk.