and no "You're no Hillary Clinton" moment, either
The one thing Biden could have done from that tight corner was to call Palin on her pretensions of being anything near the equal of Hillary Clinton in terms of genuine qualifications for the job of President. In other words, you're right, in that tight corner he'd have gone down in flames if he'd been perceived as saying that she's in any way inferior to a man, or that her gender disqualifies her; but ranking her against another woman, and particularly Hillary Clinton, would have been acceptable.
As Amy Poehler playing Hillary Clinton on SNL said, "It's not sexist to question the qualifications of a woman" who's running for office. It's sexist to not question her qualifications.