Looking through the readme, it seems this is a "matching decomp" type project which does not bundle assets (i.e. those get pulled from your local copy during the build).

I'm no lawyer, and this is no claim that it is/isn't one way or the other, but Super Mario 64 had a CC0 1.0 licensed decomp in 2019 with PC port in 2020 and Nintendo vehemently chased compiled copies of the game being shared and videos of the ports on YouTube but never went after the actual source code repo for either the decomp or port (which do not contain any of the assets). Of course there is nothing saying Nintendo can't wait 6 years and then issue action (just look how long they put up with Yuzu/Ryujinx before going after the decryption and other arguments just before the Switch 2 launched), but they were certainly aware of it when they took action against the resulting binaries/videos and didn't try to touch the code repo yet for one reason or another.

I expect some big court case to happen about this style of project within the next decade. Maybe not as big as Google LLC v. Oracle America, Inc., but still one that makes the news a fews times and gives direct precedent rather than comparisons to similarish cases.

It's not legal unless the person had the rights to begin with. It may be legal for a clean room reimplementation, but not a decompilation project like this. iD/Apogee can totally request a takedown, so I wouldn't recommend republishing that...