This one is a doozy to break down, because so much of what putin says is just utter horseshit. A brief summary of the speculation: There’s the obvious - this is Putin projecting strength to his people by reinforcing the nuclear threat they still hold over the world, this is Russia signaling specific terms under which they would deploy their nuclear arsenal (the quiet part is that they could false-flag those conditions. Obviously Ukraine would never use a dirty bomb; they dont want to poison their own land and they gain nothing by denying russian land to Russia. But Russia could set a dirty bomb if the war starts goes badly enough for them!) - and the less obvious, like the speculation that this is an attempt to softly force Ukrainian strategy away from using weapons that could be dirty bombs (or convincingly converted to them) in order to retain as much plausible deniablity as possible in the even Russia does fake a dirty bomb and then attempts to sue for concessions or face retaliation.

(Really it’s unlikely to be the grand elaborate scheme just based on putin being a dumbass, but it can’t be discounted completely.)