I agree with your claim, but I don't think it is a good idea to claim that Chromium is not "truly open-source" because (you claim) it is hard to contribute. This may cause more confusion about what "open source" really means. In fact, it's perfectly OK for open-source software to refuse outside contributions altogether! (Aka the "cathedral" model, see https://en.wikipedia.org/wiki/The_Cathedral_and_the_Bazaar#....)

In other words, open-source software maintainers are under no obligation to accept outside contributions. The only freedom you are guaranteed in this respect is the freedom to start a fork.