> You don't even need a C program to reproduce

Indeed, touch is built upon the POSIX file API (unless Apple modified it). The idea for the Apple bug report was to show it happening at a low level and not specific to a given tool.

> And the "cd" is optional, I get the same error from my home directory

Yes, /private/tmp is just an example but I'd be cautious saying the same happens everywhere. You won't be able to reproduce the issue if your current directory is /usr.

/private/tmp, your home directory, etc. are all "firmlinked" to /System/Volumes/Data in Catalina. /System/Volumes/Data/private/tmp exists but /System/Volumes/Data/tmp doesn't exist. This shouldn't really be an issue as the idea of firmlinks is to make the /System/Volumes/Data invisible and thus you should be able to relatively go back up to the /System/Volumes/Data and be transported back to the root directory, where you can find the /tmp symlink (and indeed that works fine with `ls`). Evidently that doesn't seem to work properly for file operations.