Our concern was not that our contribution would be GPL licensed, our concern was that we didn't have permission under GPL to distribute the contributions even if we were fine with them being GPL licensed. For details as to why we were concerned, see https://www.gnu.org/licenses/gpl-faq.en.html#GPLPlugins - for any company that should be a clear signal to stay away.

An employee of a company saying it's fine in an email is also not issuing another licence. We did suggest they use another licence than GPL, which they eventually did after months of arguing with them that just saying "it's okay because we don't care" makes no material difference to us and that GPL does not unambiguously provide us permission to do what they want us to do.

Hah, yeah, an employee saying that without executive authority would be a big no.

But according to this [0], I read it as quite clearly not allowed to GPL a plugin for distribution within a non free program, even if the plugin was unmodified.

https://www.gnu.org/licenses/gpl-faq.en.html#GPLPluginsInNF

But that is contingent on whether it is one work on not, which in GNU's speculation depends on how it's linked, which is not clear how to understand this for Python, and the intimacy of the communication, which is not something which we can objectively measure. We believed it would likely be one work, the plugin’s author believed it would not be. All of this was just months of wasted energy, spending arguing about licence terms instead of coding.