It is easy, even if interoperability is enabled, do not send messages out of Signal. It would be your option. But other people with non military-grade privacy requirements could benefit of improved privacy when it sends messages to whatsapp users from signal app because signal app is foss and signal would enforce better security and privacy than whatsapp app. Signal would gain traction and it could reach more people willing to abandon Meta and corps.
Best of luck also for your next fork. Please share with us your improvements in metadata privacy.