Hmm, the person tweeting this (as far as I can tell, they're not living in China currently, and is some 1st gen Chinese Canadian YA author who writes Chinese history-inspired fantasy/SF) provided some "elaboration" on the alleged situation:-

The source is from a Chinese lawyer. The law about "ruining a military marriage" specifies committing bigamy or cohabitating with a military spouse, and cohabitation is currently defined as "living as if husband and wife"

If the court wants to charge these women they are then recognizing that two women can legally have a relationship as serious as that of a husband and wife

(responding question whether the cheating couple is being imprisoned) No the soldier is threatening to sue the women unless they give him 200k RMB (27k USD) but they are threatening to counter-sue him for extortion. So right now it's just threats. Even the lawyer I saw this from doesn't know how a court would rule in this case.

Not sure about the anonymous "Chinese lawyer" source that they're relying on, and I can't find a news source reporting on this case; elsewhere in the thread they posted an SCMP article, but it was about a "coventional" heterosexual jody case from earlier this year). Be that as it may, on first glance, the purpoted legal logic doesn't seem to be completely without legs to me? If cohabitation is legally defined as "living as if husband and wife", it seems at least arguable (not saying that it's an argument that Chinese courts will definitely accept) that the court cannot legally recognise a "lesbian cohabitation" situation without first recognising the concept of a marriage/legally-recognised union between two women?