The maid mention his distain for females, so she probably assumed it'd be easier to get closer to him as a guy.Pretending to be a trap to become the disciple of a blind guy who wouldn't be able to tell even if he caught her naked in a lucky pervert situation.
I don't think her being a boy or a girl really would've changed his decision one way or another. So who knows for how long that bit is going to go on, or if the MC already knows she's actually a girl and is just going to ignore the lie because, again, it doesn't really seem to matter to him regarding whether or not she's his disciple.