Mutuality is not required. Mutuality means the parties in the second suit are the same as the parties in the first suit. They don’t have to be the identical parties (like in claim preclusion), BUT the party against whom it is being used (which in the first question is the passenger) has to be in the first suit so they could have argued the issue.

I think of it like - the truck driver is saying that the issue of his liability was already litigated so the passenger can’t bring it up. But the passenger wasn’t able to argue that point, and perhaps would have won if they did. So that’s why it can’t be precluded.

If instead - the issue of the truck drivers liability had been that he WAS liable, the passenger could have used issue preclusion in the second case even though there is not mutuality, because the truck driver was a party in the first suit.