Yes — plausible in the sense that what you heard (Chinese nationals joining the war) has support. But the version you heard (“Chinese join the Ukrainian army in large numbers to get Ukrainian citizenship”) is not supported. The documented phenomenon is Chinese nationals joining the Russian side (at least according to Ukrainian intelligence), with some lured by the promise of Russian citizenship, not Ukrainian.
It's quite common for rumors or sensational posts (especially on some BBS or social media platforms) to mix fragments of real news with exaggerated or completely fabricated stories — sometimes for clicks, sometimes as scams, or even as disinformation.
If you ever see claims like “you can get citizenship or a large bonus by joining a foreign army,” it's best to treat them as highly suspicious — both Ukraine and Russia have strict controls, and foreign enlistment almost never leads directly to citizenship.
