That's entirely BS. To contribute to these projects (my experience is contributing to Git), you need to respect a dozen conventions that seem to come from another age. Just subscribing to the mailing list is not trivial for somebody in their twenties that never had to do something like that: it's the sort of things that's easy in retrospect, but the UX is hard to discover and the lack of parallel with other tools we regularly use (such mailing lists aren't a thing that most devs use) adds a huge amount of friction.
And then you need to find somewhere that explain the conventions to try and contribute and figure out how to configure your email client, how to get a patch for your commits, how to insert your patch in your email, how to write an acceptable email subject and an acceptable email body and how it relates to your commit message, who you should CC, how to handle multi-commits contributions, how to answer emails (while respecting another half-dozen conventions)...
It's not impossible, but there's a dozen things you need to figure out, half of them you don't even _know_ you need to figure out, so it's a lot of friction. This friction might be a good thing (that's another debate), but saying "you just need to be able to send a plaintext email" is completely false and dismissive.
