While I give credit to the guy for what he has achieved, I'm not a fan boy of Elon Musk. I'm also not a blind critic of anything he does.
HOWEVER, if I was trying to deal with a situation that had never been encountered before and obvious solutions were not at hand, I'd be looking for ideas from ANY quarter. Sure you can get some weird and stupid ones, but have someone with the knowledge to triage and assess them, in case something unknown and/or unorthodox presents itself.
I'm sure that there would have been more than one person consider a mini-sub, but Elon Musk not only shared that thought, but had the resources to act quickly ... and he did. The fact that it did not get taken up, is irrelevant to me. Communication in any endeavour is often the key challenge and to compress that into a short time frame means any solution presented is going to have some elements that are going to be less than ideal.
Certainly one thing such a device could have aided was the isolation of the occupant from those involved in the rescue - something that was made very clear as being a necessary precaution.
Kudos to Elon Musk for having a go (IMO) and I would not be surprised to see a bit more development on this project, to at least get it to a practical operational stage.