I wish I knew the real story. Anton was so ridiculously excited up till then that I find it impossible to believe he just couldn't be arsed to make an effort. Maybe it was her fault for some reason, but even if so, it was wrong of him to blame her out loud, regardless, and especially for being incapable after she did such a relatively good lively jive. We know she has the stamina and enough chops to make a go of a reasonably-basic cha cha, so his un-gallant declaration doesn't seem to hold a grain of truth.
It's aggravating me so much I'm reassessing my admiration for him - looking back, he found it suspiciously easy to make fun of his partners and he's also often quite patronising and smug with "his girls."
I went through a similar reevaluation with Brendan last series - his obvious disdain for Sunetra was so rude, bordering on cruel - wish I could see his face when Aliona lifts her SECOND mirrorball, especially after he's said such rude things about her.