"If you contribute to Libpijul, we are asking you to allow us to reuse your work, including under a different license than the current GPL-2.0-or-later."

Not sure why this is news (and the title is misleading: this is for Libpijul only, not Pijul nor Sanakirja).

This has had 0 impact on contributions so far, since "signing" here just means including a patch as a dependency. In 100% of the cases so far.

Also, Libpijul is probably the last place you'd start if you wanted to contribute to Pijul. And since Pijul is GPL-2 and has no CLA, this means there's nothing hidden.