Nukes are probably not Russia's strongest offensive resource, and certainly will not be the first resource to be drawn upon.
Compared to those things, the power of nukes is trivial; all they can do is destroy. You can't surrender to a nuke, and the point of warfare is to convince the enemy to submit, not to damage the ecosystem. Damaging the ecosystem is just a side effect, so it's the wrong way to measure the strength of offensive resources.
So, it was an impressive threat. I doubt he actually would follow through... but he might. That's his leverage - that we wonder if he might. Rationally, though, it's a crummy threat, because rationally his own downside is far too big.