I don't know if he's dismissing UB's ability
to win the Derby because she's a GIRL, so much as having the philosophy that it's not worth throwing a filly of her quality into that 20-horse madhouse of a race.
If the glory of winning the Derby isn't THE most important thing in your life - everything else be damned (including the horse's welfare?
) - then the importance of running a filly in it drops pretty far down the list.
A colt winning the Derby has a greatly-increased value as a stallion. A filly has no such increase in value.
IMHO, Unique Bella already has enough black type (and the royal pedigree
) to cement her value as a potential broodmare.
It may be possible that the big grey gal will take on the boys in some future race, but it's sure not going to be in the Derby.