False. Look at https://osgameclones.com and projects like FreeDoom. You must be young and it shows how disconnected are the new generations on libre reimplementations.
The fact that these exist does not mean that they're immune from legal challenge. If the original creators wanted to sue, there are all kinds of claims that would have a decent shot in court (e.g. trademark, trade dress, design patents) besides "you copied our copyrighted source code." The clones exist more because people are being cool about it, and because there's not a strong economic incentive to challenge them. Those things can change at any time.
Out of the two claims, the only one that made it to appeals court was about whether it was fair use for Bleem to use screenshots of PS1 games to advertise its emulator (which was compatible with those games). The Ninth Circuit decided it was. But that's not relevant here.
The other claim was more relevant, as it was an unfair competition claim that apparently had something to do with Bleem's reimplementation of the PS1 BIOS. But the district court's record of the case doesn't seem to be available online, and the information I was able to find online was vague, so I don't know what exactly the facts or legal arguments were on that claim. Without an appeal it also doesn't set precedent.
If there were a lawsuit over OpenTTD, it would probably be for copyright infringement rather than unfair competition, and it would probably focus more on fair use and copyrightability. For fair use, it matters how much something is functional versus creative. The PS1 BIOS is relatively functional, but a game design and implementation are highly creative. On the other hand, despite being creative, game mechanics by themselves are not copyrightable. So it might come down to the extent to which OpenTTD's code was based on the reverse-engineered original code, as opposed to being a truly from-scratch reimplementation of the same mechanics. Visual appearance would also be relevant. Oracle v. Google would be an important precedent.
FreeBSD, NetBSD and OpenBSD at first when every BSD OS was just part of 386BSD it used to have AT&T code. That code was rewritten replacing every propietary part and after that (and noticing BSD 4.4 was incomplete) we got clean FreeBSD, NetBSD and OpenBSD from a NetBSD fork.
Another similar case with exact grounds was GNU which with Linux it completed an OS albeit in a hacky way, because the original OS would have been GNU+Hurd, but both are reimplementing Unix. Same SH derived shell, but extended. Kinda like OpenTTD. We have GNU Coreutils, Findutils, GNU AWK reimplementing and extending AWK (even when AWK was propietary), GNU Zip, Tar... the list goes on and on.
Oh, another one: Lesstif vs Motif. Same UI, if not very, very close to Motif 1.2 in order to be interoperable. Today it doesn't matter because nearly a decade a go Motif was relicensed into the GPL, but tons of libre software depending on propietary Motif was just seamlessly running with LessTIF libraries except for some rough edges/bugs. One of the most known example was DDD, a GUI for GDB.
