last night I actually watched a lecture on nuclear deterrence and why Russia hasn't used them yet. One thing that came up was the self-use/testing within their own borders as a show of force.
After all, they could just nuke Ukraine/Kyiv and be done with the war, but it's more likely China/India talked him down from the escalation (but there are three main caveats)
So it's kind of interesting to see him say this now; we were waiting for a while to see what nuclear-rich Russia would actually do, since incursion onto their territory (Kursk).
The overall consensus was a drill/test to display that they still have the capability to do so.