My understanding based on my experience is that “anyone” in the submission model means “any project members”, as opposed to specific members (project managers, project owners, or just specific people, like in this example) .
Of course, that’s just based on what I’ve seen when I try to add obvs to projects. I could be wrong, and maybe it is broken & joining is a workaround, not a solution.
But based on Marina’s reply as well, I think you have to join. I don’t recall ever (since 2016) being able to add observations to projects I haven’t joined, nor seeing any complaints or bug reports (here or under the old Google group).