But OpenTTD is explicitly a faithful copy of the original. It replicates the original product in appearance and behavior and is open about it. If you were to dig into source code history, mailing list archives, chat logs etc. I'm certain that you could find a lot of evidence to support this position.

"Behavior" isn't copyrightable; it explicitly isn't, in fact.

To what extent did they copy "appearance" other than supporting the use of the original assets?

It is certainly possible that they didn't scrupulously maintain clean hands, but I wouldn't automatically assume that.

GNU’s Not Unix is explicitly a faithful copy of UNIX. It replicates the original product in appearance and behavior and is open about it.