Earlier quoted context omitted.
Curious to know how and where you got that impression. There is no such requirement. There are similar private requirements for the cartel known as NAR. What this means in practice is that pocket listings actually make to to the MLS for one day before apparently being sold "in 1 day". Zillow as property owner is not part of NAR. Zillow as broker is also not part of NAR, although if they were, they'd do exactly as ind…
I think MLS would like to give you that impression. They seem to function as a cartel of some sort. (At least I have never understood their purpose.)
Today they essentially function as data brokers and unofficial labor organizers, collecting membership dues from agents in exchange for ensuring that the agent's name is on their listing wherever it's shown online and (along with NAR) pushing agents to keep "standard" commissions on their listings (5-6%).