“no Western type of military can really do this sort of combined arms operations at scale, with the exception of the United States. But even the United States Armed Forces would have a very difficult time breaking through these defensive layers because no Western military in the world currently has any experience in breaching the types of defenses in depth that the Russians put up, in the south and east of Ukraine.”

That's the crux of it. Russia had a year to dig in. It's very hard to quickly make big gains as a result.