You don't have to overrun Moscow when you have missiles that would reach it in 4 minutes.

Medium range nuclear missiles in Ukraine were a real possibility since Trump pulled us out of the INF treaty in 2018 and Zelensky explicitly talked about Ukraine getting nukes a few days before the invasion.

The US has no policy against using tactical nuclear weapons. Russia felt threatened exactly as the US did when Russia put missiles in Cuba after the US put missiles in Turkey.