Lets be real. If it was nuke, russians would respond with their own nukes on frontline or worse and it would lead to more nukes and escalation from Ukraine/NATO (why would they drop single nuke only?)
And if they didn't respond to nuke attack with their own nuke (just like they didn't to this attack in reality), it would be over for russia, nuclear power not responding to nuclear attack on them have many implications none of them are good for them