You are missing a "?" and the answer is no. It isn't even Open-Source, although it uses open-source components (MIT licensed).
Hmm, did you maybe not read the whole post? The formatting almost had me thinking, it stops after the first paragraph.
The post does acknowledge that BeamNG is not open-source, but it copied Rigs of Rods, which is free software.
The article is written with the same speculative insights as an angry manchild before breakfast, but they do correctly spot that Thomas Fischer was a contributor to Rigs of Rods, and decided to go in another direction from what he had learned there.
To call it copying is quite harsh, since he did change the engine and did write large parts of it.
It would be like calling LibreOffice "stealing" Microsoft Office, when one just served as the inspiration for the other
There isn't anything particularly wrong with copying though. And copying isn't stealing.