I am pretty sure this just means that messaging companies will need to provide a publicly accessible API. There won’t be any mandate to actually implement this interoperability beyond that, but of course someone could write an open source client that uses all these newly exposed APIs to make a unified chat app with Tinder, Slack, and WhatsApp all in one place. But the tech companies themselves aren’t being forced to work with each other, the ask from the tech company side is actually relatively small.