The vast majority of what's showcased on that site and playlist were done via assembly hacks, not c.

Super Mario 64 decomp has only publicly been available since late 2019. And even then, there's still holdouts as the toolset for ye olde ways has been around longer.

Couldn't find it but, did they fix that one ghost coin in little-tall island?

I assume not, since that would require an asset change, and the game assets are not included with the decompilation project. It's also not a goal of the project to change existing behaviors of the game (including bugs)