In what instance can a divorced couple file joint taxes?

r/

Location: Texas
A man I have been seeing says he has been divorced for 10 years. He and his ex-wife still file taxes jointly. I thought this sounded like a red flag but he is angry with me that I don’t believe he is actually divorced.

He has produced divorce papers to prove it but still files joint with his ex-wife. In what instances can a divorced couple do this??