There are other red flags in the terms, and not the comrade kind. https://www.solidarity.tech/terms
And then its closed source. The polished aesthetic gives a vibe coding vibe too. But thats pure speculation from my side.
In general i makes me wonder if there ever be good tech under us law again... Maybe its just a long time since i read the terms of closed source projects.
Zetkin is open source, run by a foundation. I like the explicit values mentioned: https://zetkin.org/en/foundation