alt Hacker News"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.
Show a set of random persons gameplay video clips from TTD and OpenTTD in its default settings and ask them which one of the two games they are watching. They'll be struggling.
It is about the entirety of the product, not its parts.