But China and Russia settled their borders about 20 years ago. The PRC no longer claims Outer Manchuria so they would be going against their word if they were to seize it. The political damage can end up being pretty severe. It's just not worth it especially when you remember that the only thing that's particularly valuable in this territory is Vladivostok. Moreover, China gains a million advantages from having a powerful and vibrant Russia around so the whole idea is a nonstarter.
The PRC and the Russian Federation settled their borders 20 years ago. This is not equivalent to all future governments in "China" and "Russia" abiding by the agreement. After all, the Tang also had a border agreement with the Tibetan Empire. Does that mean China should grant Tibet independence and revert to Tang borders?
Many people are going about this the wrong way. Geopolitical interests evolve and state-to-state relationships are never static. The Russian Federation is a friend and "almost ally" of the PRC and hence the PRC should abide by its border treaties & clamp down on idiotic voices promoting revanchism.
But future states in Russia may not be so friendly. If Putin falls and the next Russian president aligns the country to the West and moves to join NATO, should China still abide by the aforementioned treaty even though it means facing a NATO state along the longest land border in the world? If your answer is "yes," then I'd also consider you foolish. Countries do not have friends; they have interests. As the geopolitical situation evolves, so must state-to-state relations.
