You're of course technically right, but I would argue that in practice Audible is also a gatekeeper due to their dominant and successful position (though of course to a lesser degree than Apple). I have family members and friends that will not even consider buying an audiobook if it's not on Audible. That's a testament to Audible's good work at making a good platform, and I believe Audible deserves to be in the position they are in. But, I do think they have effective gatekeeper status, and a lot of that is a result of their exclusivity practices. By ensuring that there can't be any other audiobook store out there that has all the books a person wants, and that you can't sideload a book into your Audible library that you purchased elsewhere, they force a high level of inconvenience on anybody who wants to buy a book somewhere else.