Chain had to let go fairly late in all that carnage. If the chain broke first the crank sprocket wouldn’t have shed all its teeth. The crank sprocket spinning against the chain that wasn’t spinning (or wasn’t spinning fast enough) stripped the teeth.
If the chain was just really loose, like mismatched parts, it could have skipped when he revved the engine. I’d make sure I checked the cam and valve train really well too though, because the other possibility is that there was too much resistance from the cam gear for the crank sprocket teeth. Which could mean the problem started on the top end.
