You're quite simply wrong - the docs do not say that anywhere.
What they do say is that you are not allowed to route loggers circularly - if you route (for example) JUL through SLF4J, you are not allowed to route that output back to JUL, otherwise messages would never be dumped to output.
Even the docs say it does not work w/ multiple loggers. But I'm not here to teach, good luck.