single line of defense. Once you're through that gap there's no one else back there. Only a couple of guys on the kickoff team had a realistic chance of tackling Alfrey on that play, and they missed tackles.
There are plays in every game where a couple of guys miss tackles. This one of course sticks out, and I'm sure the coaches weren't pleased about this one, but I don't think Houston comes away from this looking as foolish and ridiculous as you suggest.