Siberia as a whole is not considered as a part of China. Maybe less than 5% of its land is considered controversial by few chinese nationalist.CCP's long term goal has never involved siberia for the rejuvenation of china( Taiwan is) as a leading global power,and ccp has their goal set for centuries. CCP's goals is tied to aspirations regarding historical chinese glory and Siberia has no part in it.
Annexing siberia would be a political disaster,set a bad precident and provide all the ammunition toward the western coalition to vilify china once and for all.
It's very likely that China will make amends for its political mistakes toward Japan and reconcile.
CCP learns from their mistakes.
Historically china and japan had a cordial relationship,that's before the western hegemons turned the pacific inside down and set everything haywire.
When China is able to seriously dismantle or challenge US hegemony and make a move for reconciliation toward japan ,Japan will gladly accept an alliance with its historical ally.
Japan always considered China/Korea rivals far before westerners stepped foot in the region. And even period they entered their isolationist periods, Japan always invaded korea/China fearing a strong Central chinese government. They fought for centuries since. Idk where you got your info from.
Hard core CCP and chinese nationals have eyed Siberia for decades. It is one of Chinas long term goals as well as taking back Taiwan and the whole south china sea. Deep down both China/Russia consider eachother rivals. On the surface the only thing that unites them is their anti-US stance. If US were to disengage from the region which in all likelyhood they will in the next few decades they would turn against eachother in a heartbeat. Chinese consider siberia as part of a greater manchuria.