Medvedev has been posting bloodcurdling threats for months.
He threatened to nuke Sweden as they'd stopped selling Absolut vodka. Of which I believe he consumes quite a lot these days.
There is a very good reason why no one will negotiate with Putin again: quite simply because he could never be trusted to stick to any agreement. In St Petersburg he rehashed the old fantasy that Russia withdrew its forces around Kyiv last year as part of a deal with Ukraine which the latter failed to honour.
There is an interesting story in the Express from yesterday by the way:
https://www.express.co.uk/news/uk/1796679/ukraine-soldiers-training-uk-russia-war-crimea
A couple of days ago Ukraine hit the Chongar bridge, one of a couple that links Crimea to mainland Ukraine: slowing Russian logistics there even more. Who knows if the "sources" quoted in the Express are accurate.