A lot of people are bagging on this secret because of the phrasing, and I agree it could have been better, but I think the concept itself has possibility, especially because...
...Kyubey exclusively targets girls for contracts in canon because of the energy he believes they possess. If he were to go after a "boy" it would likely be because he thought the individual had "female" energy, and therefore maybe wasn't a boy at all.
Alternately, Kyubey seems to like granting wishes that will some way backfire on the wisher and cause despair. A boy that wasn't actually a trans who made a wish to change himself for whatever reason might regret it later on, which could play into Kyubey's paws.
Possible Fridge Brilliance?
...Kyubey exclusively targets girls for contracts in canon because of the energy he believes they possess. If he were to go after a "boy" it would likely be because he thought the individual had "female" energy, and therefore maybe wasn't a boy at all.
Alternately, Kyubey seems to like granting wishes that will some way backfire on the wisher and cause despair. A boy that wasn't actually a trans who made a wish to change himself for whatever reason might regret it later on, which could play into Kyubey's paws.