This is exactly the problem. If they support interoperability then they will allow their users to continue using the Signal app which has high security standards, even if the particular conversation is not as secure as native signal conversations and they can’t control what the third-party app does. This will help grow the Signal network (because now it is easier for WhatsApp users to incrementally switch to Signal) and become more secure.
By rejecting interoperability they may be slightly improving the privacy of the 1% of users where their conversation partner would have switched to Signal, but are harming privacy the 99% of users that will now need to switch to WhatsApp for those converstions and are harming their future network growth (which would bring even more users to a private solution).
Perfect is the enemy of good
This is exactly the problem. If they support interoperability then they will allow their users to continue using the Signal app which has high security standards, even if the particular conversation is not as secure as native signal conversations and they can’t control what the third-party app does. This will help grow the Signal network (because now it is easier for WhatsApp users to incrementally switch to Signal) and become more secure.
By rejecting interoperability they may be slightly improving the privacy of the 1% of users where their conversation partner would have switched to Signal, but are harming privacy the 99% of users that will now need to switch to WhatsApp for those converstions and are harming their future network growth (which would bring even more users to a private solution).