What makes you think this means Whatsapp will just get access to signals data? If it works by someone saying "sent specifically a Signal message to this number" then the issue is that person's handling of your data. And even then, signal could just play dumb until you yourself specifically say "allow Whatsapp messages from this number explicitly".
Edit: also, wouldn't this make it verifiable that Whatsapp is actually using proper encryption?
Ah yes, I forgot that this has actual screens unlike most other foldables. Thanks for the sad reminder.