As I explained to a colleague I am not sure from which relays relay.nostr.band collects the information, its author says from all of them but I cannot verify it.
In my case the combination of relay.nostr.band together with filter.nostr.wine gives me practically all the followers.
At the moment I don't know any relay that allows the reading of "all" in the same way that nostr.mutinywallet.como does with the writing. In this case if I have been able to verify its code and what it does is to read the api of nostr.watch of relays online and to write in all of them.