An absence of the muscle groups you mention would indeed not be fit, since any person actually lacking those muscle groups would be completely disabled.
However, I don't see what that has to do with this young lady; she clearly has a good set of abs, to start with. "Leg muscles" (I assume you mean quads, hams, adductors, since we only get to see the upper part of her thighs) are on the light side, but not what I'd call underdeveloped. My wild guess is like most models, she does a lot of cardiovascular excercise, but not a lot of weight training. She's got a low bodyfat percentage, but not exceptionally so; no visible ribs despite her pose. I see absolutely no reason to _not_ use the word "fit" in regards to her.
And as for the emergency room/birth remark, don't be silly. Women tend to put on quite a lot of weight during the nine months leading up to the big day, in case you hadn't noticed.
And the word "anorexic" is not synonymous with "skinnier than my preferred body type". Don't throw it around so carelessly.