They wouldn’t likely have been able to keep a stolen Nuke sub quiet very easily. Which supposing it’s true and there was a stolen sub. It would suggest a level of secret keeping that would be decidedly unusual for the amount of assets that would need to be invested and involved in such a hunt.

But a Subs payload detonating would explain the strength of the blast. No news on mysterious radiation spikes thus far. So can’t know for certain if it was Nuclear or an above average yield of conventional explosives.