Absolutely untrue! No, make that spectacularly untrue!

I have been involved in organized sports (soccer, squash rackets, track) for a very very long time. In all those years, I have seen every kind of athlete that are fit, myself included, cramp at times. Maybe being out of shape contributes or not, but to say you do not cramp if you are in shape is definitely untrue. Watch world-class soccer sometimes (the last european championship, for instance), and you'll see every kind of fit specimen cramp at times. Especially in long extra-time games.