The word 'fact' is taking some abuse here.
Nobody has yet been able to explain to us how exactly the Russians are going to rearm. Yes, they've got lots of raw materials, some of which are difficult to access and require Western tech (not Chinese) to access. They're going to need more than that. How is this going to be accomplished in the face of the sanctions? Remember they'll need to do more than simply acquire a few weapons here and there.
There's also the issue Russia face with soldiers and the army culture generally. They do have a bigger population than Ukraine, it's true. They also have to contend with already massive losses and they're currently losing a general about once a week. That sort of expertise is not quickly replaced. Conscription was already something that Russians with options sought to evade, in no small part because of the horrific hazing systems. The prospect of dying in Ukraine is not going to make that any better. And they have a fossilised, Soviet era military culture plus a massive theft problem in the army. Morale is low, which is all bound up with the failure to look after their equipment and do things like routine tank tyre maintenance. Again we're talking about a significant military culture change that would be necessary. Where's this coming from?
Actual defence experts, people whose understanding of the Russian military goes well beyond Roxburgh's, have provided very detailed analysis on Russia's problems here. I linked to some upthread. It's not as simple as got lots of people and lots of resources so no problem.
On the separatist armies point, those look like rather less of a threat than they did before the invasion. Because the Russian invaders have treated Russian speakers and ethnic Russians badly too. There are Ukrainian politicians who were previously pro Russian who are now, well, not so much. So these armies would have to actually exist before they could be armed, which becomes less likely when you've been stupid and venal enough to commit a shitload of war crimes.
Basically, if people want to argue that it's inevitable that Russia will be able to keep an actual war going to the extent that the West's current tactics are doomed to fail, they're going to need to do a better job at explaining how. Roxburgh didn't, and neither has anyone in this thread who agrees with him.