Training is an on-going process, and 3 weeks is not a long time to be working to achieve a rock solid leave/give.
It sounds like you have been making fantastic progress, and I think the lure of a spare rib bone would be a lot for most dogs to avoid.
I truly understand your worry at the situation, but try not to let it disrail your training... in a few weeks time you may see another rib and your leave will be proofed enough to avoid the temptation.
Even if he did eat a cooked bone, once he is fully recovered from his recent trauma, it is unlikely it would cause the same damage - it sounds like he was incredibly unlucky as, while there is always a risk with things like that, many dogs every day do snaffle things they shouldn't without ill effect.

Don't let the experience dishearten you - you know how to approach the problem, so don't lose confidence!
(And FWIW, a muzzle used as an interim preventative measure isn't a bad thing - the problem really comes when people use a muzzle *instead* of training a leave... if you really are very anxious, I would go for a muzzle while you are out, but continue the training... and do remember to bring the training out onto walks - set up situations where he can "leave" low value things you have planted so that he learns that the training you are doing applies out on walks as well as at home!).