I'd say they're full of it.
You aren't going to be pulling the wheel forward (out of the dropouts) any more than you would with a similarly set up geared bike.
That is to say; I've pulled my rear wheel out of the dropouts before, but usually on my geared bike with QR's, and usually from a stop, on a hill, in a small gear...
Riding fixed won't make that situation any more likely to happen, or make it any more dangerous if it does.