They’re using their newly acquired license to TTD to hold the better remake hostage, presumably in the hopes that enough people will pay up. You can still play OpenTTD for free if you already have it on your account or download it directly from their website: https://www.openttd.org/downloads/openttd-releases/latest
You can also donate to OpenTTD, as they will receive none of the money you give Atari: https://www.openttd.org/donate
OpenTTD doesn’t use any assets from TTD, all code, graphics, sounds etc are original. That said, it does aim to replicate TTD closely and has far surpassed that.