I didn't say a trademark isn't a form of IP. I said their application for a trademark was rejected due to "open source" being too generic/descriptive, not due to the reason you directly asserted above ("They don't hold the IP, I don't really see any way they could be granted a trademark on it").

You can read more about this at https://opensource.org/pressreleases/certified-open-source.p... or https://www.techmonitor.ai/technology/open_source_initiative... among many other sources. Or a much longer blog post from a lawyer who is active on HN: https://writing.kemitchell.com/2020/05/11/Open-Source-Proper...

fwiw, a non-OSI attempt to trademark "open source hardware" was also rejected for the exact same reason. https://opensource.com/law/13/5/os-hardware-trademark-reject...