My issue with this argument is that I'm not sure how much of OpenTTD is their IP. OpenTTD has been development for so long that I doubt that any original disassembly remnants remain in the latest version of OpenTTD. The only true piece of IP that OpenTTD may use is the name (the TTD part of OpenTTD) and the graphics, the latter of which being the more important one. However, as far as I know, OpenTTD devs have created their own version of all the assets that are also much higher resolution compared to the original. As a result, I see OpenTTD as an entirely separate game, that's been heavily inspired by original, but is its own separate entity.