Merely being “listed as a possible witness” doesn’t really mean anything. And the case is closed, I don’t think Joe was called.
What it means is that Joe likely had knowledge of the case. This was about the Tribal Bond scam that Archer, Cooney, Galanis and other partners of Hunter used the Biden family name and influence to scam investors. It reinforces the claims that Joe was more familiar with Hunter's business deals and his partners than he admits.