It looks female to me. The facial bone structure, and the way the armor fits on the chest, not a very curvy female but female.
Oh, I can see it/why people think its female...and I myself would probably have gone for slightly thicker/muscular biceps and thighs...It's the broadness of shoulders that's the give away for me...and the chest looks more "pectoral" than "mammary."
Now, if we're going with "flat chested"...I guess it's more possible...or if the artist was going for "deliberately indeterminable" then it works too. I mean, I can totally see, I think, what other people are seeing. It just wasn't what I saw first.
And, just for the record, there is a looooot of ground between "no chest" and "huge with lots of cleavage"/ridiculous falling out of her shirt chest that could easily have conveyed "female" without being overtly sexualized but still clear.
Bottom line...there should have been a Generate Random Appearance Traits table for this entry: Wings? No wings. Horns? Big or small -pointy or goat- or ram-like? Cloven feet: 1 or 2. Snake tongue. Extra fingers/toes. Extra eye. Snake like neck. Skin color. Eye color. etc etc. to really subtle stuff.
I had a cambion in my campaign world was fairly normal looking except for the "glowy fiery eyes" when he got mad and his left forearm was actually a think serpent-like body splitting into three individual snakes from the wrist.
Stuff like that should have been immediately possible, out the gate, built into the MM entry, with these guys. These aren't just "demon type X"...whose appearances should all looks the same...and they aren't all "offspring of succubi/incubi"...if they wanted the appearance tacked down, then they should/could easily of said, "they
are the offspring of succubi/incubi"...end of story.
I suppose the nod to "Spawn of Grazzt" is supposed to give us the idea that they can look different...but I don't think it goes far enough.
These guys are supposed to be manipulating and creating chaos...possibly taking over/running towns behind the scenes?!...Who's going to let this thing live to see its first mouth if it's born bright red with little horns on top?!...forget about the tail or wings! And if it lived, how/why's anyone ever going to listen to them...unless they get charmed? The creature, stat wise, looks perfectly fine. But the fluff as written seems a hard sell.