You are correct that DIDs are not necessary for Signal and WhatsApp to agree to talk to each other, and to make such an interop project succeed.

However, I don't think the requirement captures what's truly interesting here. As stated, the requirement is satisfied by two companies agreeing to interoperate. What DIDs do is take this out of the hands of the companies and put it into the hands of the DID controllers. I can engage in secure communication over email or SMS or whatever channel I like, without the cooperation of the software vendors I use -- and then, because I created the channel that provides the security, I can take that channel with me to a new environment without the vendors' permission or cooperation. In contrast, when Signal and WhatsApp agree to interoperate, they have not really freed you from a silo. They've just made the silo bigger. The silo still depends on their terms of service, the ongoing cooperation of their systems, etc.