I'm finding it hard to believe a thread can be stripped by pedalling backwards. Were you using a lockring? If not, is it not possible your cog just unthreaded itself?

Guys such as Chris Hoy put about 550 watts through their pedals from a standing start and never strip cogs. Locking a wheel with your legs needs only about half that wattage.

An old aluminum Dura Ace or Zeus cog might possibly strip - I've seen a tooth ripped off an aluminum cog.