While true in some aspects, it's not true in others. Most of todays "modern" open source ecosystem live within GitHub which is not open source, using services for development and infrastructure that is also not open source. While many developers local tools tend to be open source, the rest of the infrastructure we use tend to not be open source. Even many of the biggest package registries for open source are not open source themselves.
I'm not saying 100% of the open source community leverages these services, but a lot sure do.
I agree with your top most point the most though, there is no clear path for designers to actually contribute. And if there is, it's a pipeline of "come up with requires -> design -> code", whereas contributors are not as excited about the last point if someone else did the steps before. Hence we see more code contributions than design ones.