Such a statement doesn't make sense. Not to mention that you can't retroactively un-license something (unless the license has a termination clause). Free software licenses do not have termination clauses (by definition, the FSF won't approve them if they do because it restricts practical freedom). Though I'm fairly sure it would count as an "open source" license under the OSI definition.
> In most cases developers sign no agreements when their patches are merged.
If you submitted code under the DCO, you've signed an affadavit saying that you have the right to license the code and that it is licensed under the license of the project. For copyleft projects, you don't even need that (if you've distributed it then you've implicitly licensed it under the original license). But in either case, once you've licensed you code under some license then you can't terminate that license if it's a free software license.
The world would be a much different place if smart people in the past hadn't invented Copyright and Patenting, which both are mostly being misused and are not used to foster innovation. Back then it was normal to hold onto secrets and patents and copyright were invented to incentivize publication under well-defined and not too long exclusivity periods after which other can freely improve on it. But today we have 20 year patent monopolies and Disney copyright absurdity.