Omfg, don't act so dull. OpenTTD is an asset-compatible reimplementation of Transport Tycoon Deluxe. The string "Transport Tycoon Deluxe" is in the source code/resources, it is right on the top of the projects website:
Yeah it's almost like they're very intentionally making their project legally distinct by calling it just "OpenTTD" instead of spelling it out. Please don't call me dull when you are apparently too fucking stupid to grasp this very common practice in fan projects.