I dunno, encased in concrete and buried in the ground sounds unsually coincidental to me. It's possible it just laid there dormant until the radiation leaked, caused some catastrophe, and DHARMA set up the Swan to regulate it. Still feels like a piece there is missing.
The explosion when The Swan blew up wasn't big enough for it being a Nuke if it was. That would have done a lot more damage, not to mention radiation that no one suffered from when it blew. I seriously doubt there was a nuke under there.
I agree it doesn't quite match, which is why there's probably a piece missing, something else happened, but I seriously doubt that it's just coincidence that we have these two things, a hydrogen bomb, and a something else electrocmagnetic, BOTH encased in concrete and buried underground.