Indeed.Neither were the Russians evidently. What an extraordinary statement of intent this is.
For context, the distance from Chelyabinsk to Severomuysk is 4000 km. Chelyabinsk (where the tank engine factory was blown up a few days ago) is itself 1800 km east of Moscow. How on earth did Ukraine manage it? What route did they take? Through Russia? Or some other route? Did they have help?
I read somewhere a few weeks ago that Ukraine, being in what is for them, an existential war, are aiming high and seeking regime change in Moscow. One of their objectives is to foment unrest in Russia, to show Russians that Putin cannot protect them. Could it be that Ukraine has help from friendly partisans in Russia's ethnic regions?
According to a Russian census in 2015 there were 5.8 million Ukrainians living in Russia. Naturally numbers will have changed since the occupation of Crimea, Donbas, and then the full-scale invasion. Still, I wager there are still very significant numbers there. It only requires a small percentage of them to have the desire, capability to liaise with Ukrainian military... and there we go.
And/ or, as you say, friendly Russian partisans, or Ukrainian saboteur groups.
Maybe we'll find out in a book one day?