I don't think there's anything "special" here. You have the same problem currently where finding the canonical location of a repository is done via some out-of-band social network or website.
On GitHub, you also can look at the stars to give you extra confidence, and on Radicle the equivalent is the seed count for a given repository.
Then why does the documentation say this is "important"? GitHub certainly does not have a notice anywhere saying "it's important to only publish repositories you own or are a maintainer of" (...well, I guess it could be buried deep in some user guide I never read, lol).
I think it's currently more likely to happen on Radicle given there is no search or discovery functionality, and repositories exist on a flat hierarchy, ie. they are not namespaced by user/org name, so harder to distinguish if they share the same name and description.
Why are those items not included? Being able to browse one org/developer's repos is a very useful indicator when investigating a new unknown repo/project/org/person, trying to determine if the risk of time investment is worth the effort.
On GitHub, you also can look at the stars to give you extra confidence, and on Radicle the equivalent is the seed count for a given repository.