No. Some very large companies have to offer interop for their messageing platforms, smaller ones don't. So if Threema wants interop with Signal, Signal can say

No. Some very large companies have to offer interop for their messageing platforms, smaller ones don't. So if Threema wants interop with Signal, Signal can say yes, no or they can discuss it. If either Threema or Signal want to interop with Whatsapp, Facebook has to offer that, because Facebook meets one of the two size criteria.