Less so than standing up. Following aircraft you'll need to pan around and that means either you, or the lens, will be swinging around too. If you're sitting down, you'll have to swing the lens and a monopod would just get in the way. I think they were talking about sitting down between acts.
From my experience, if it's a windy day a monopod would be a good idea. Every (all 4) air show I've been to in the UK (Duxford, Blackpool, Shuttleworth and Sywell - August-September timeframe) has been windy, with gusts of 25 mph or higher. Panning against that, hand-held, is a real challenge. By comparison, the spring and summer airshows in the US feature much calmer weather and hand-holding is limited only by your stamina, although while using my 500 f/4 I'll use a monopod and waltz around it.
And using mode 2 on the IS on my 70-200?