I'm not so sure about that. The Chinese could do much more with Siberia than the Russians are doing. To begin with, it's actually possible to grow rice in the Ussuri and lower Amur Basins. The Chinese do so on their side of the border. The Chinese could easily settle 50 million people just in Primorye, Khabarovsk and Amur Oblasts--comfortably. And probably another 5-10 million in Chita and another 30 million in Sakha and another hundred million from Irkutsk to Cheylabinsk in the Urals.
There is a lot of potential land in Siberia that could be farmed with a crop such as Quinoa that is adapted to a cool climate and short growing season. Much of Siberia's soils are podzolic, but they are not the muskeg swamp you get in the Canadian North except in West Siberia, and even there, there is good land alngside the Ob, Irtysh and Tobol Rivers and alongside the Yensei. The alluvial soils between the Lena and Vilyuy between Yakutsk and Zhigansk are also reputed to be quite rich.
It's more of a cultural and economic thing with Russians. A. Russians tend to be uncomfortable living outside of Russia and B. Most of Siberia fell into complete neglect after the fall of the USSR and the infrastructure completely deteriorated, though Putin is trying very hard to bring it back, especially with the railroad and road he's trying to build to Alaska that finally made it as far as Yakutsk. The one major exception is the oil and gas patch in the Ob Basin around Surgut and Samlotr and Khanty-Mansysk and then north to Urengoy in the tundra. That area apparently still is growing despite the terrible climate. But people will go anywhere for oil.
And then there's global warming, which is likely to change everything in Siberia. This year, while we're freezing, apparently, Siberia is experiencing record warmth. Permafrost is melting over there, which is making more land arable and making more arable land able to support more varied crops, just as in Alaska. And because of the methane tied up in the permafrost, we are already into a feedback loop that is making the warming self sustaining. Already methane bubbles are being detected coming up from arctic lakes. So Siberia can support more people than it is supporting now and is likely to support even more people. At some point in the future if Russia dosen't make use of it and fill it up, China is likely to, especially if sea levels rise and China loses some of it's best land.