This rule is in response to an observed problem when operating at scale (most of this protocol has been tested in reality).
Patches *are* invited from everyone, and merged quickly.
What often happens when you have 30+ people working on the same set of problems is that multiple people spend time on the same problem without telling each other, and everyone who tries to send a PR after the first patch is merged gets annoyed that they wasted their time, this is a pain point that can be resolved very simply: you MAY claim a problem to work on it, but if you work on it without claiming it you accept the risk that you might be wasting your time.
This risk disappears if you claim it first.