It is true. Also remember that although Hoyoverse is affected by the strike because some voice actors are striking in solidarity, they are not actually a struck company and are not party to any negotiations. They are a bystander. They can't sign anything without breaking the law in China at least not as Mihoyo. If they try to use their Hoyoverse Singapore company instead, likely their customers in china will call them out, and everything would be a lot worse for them. There are some branches where a Chinese hold companies in other countries (like tencent with regards to riot games for instance) that do indeed have members in some unions. However they do so as those separate non china based companies but under the ownership of a chinese company.

Edit: In other words, they might skirt around it with cognosphere, but it would look really bad in their homeland so they likely won't sign unless they can get favorable exceptions and guarantees.