Sorry, I don't remember the specific instance of what happened there, but specifically Singularity was never technically "public domain". It was always copyrighted and licensed (firstly with the DOE/LBL license based on the 3-clause BSD license, then just BSD3).

Was the issue on the project copyright, or was it the copyright in the source files?

While I have some experience in open source and copyright, I'm certainly no expert nor do I claim to be. Luckily I have people I can rely on for advice and guidance (e.g. back then, it was probably LBL/UCOP's Tech Transfer), but sometimes things still get mucked up.

Sorry if there was a miscommunication or an error on my part back then.