From a quick reading of the proposal: They can do that by not wanting to. They «will have to open up and interoperate with smaller messaging platforms, if they so request» which leaves open the possibility that neither Facebook nor Apple requests interoperability from the other.
(And, I suppose, the possibility that they don't have to interoperate with other platforms that are large according to the same rule, only with smaller ones. But I guess that's just sloppy wording in the announcement.)
