@Adenoid Hynkel I can confirm I just did it again deleting the app and skipping the backup.
All your chats EVER are there. Let me be clear the CHATS not their contents.
So exactly as I explained earlier someone would be able message and continue any old conversation without having the contacts.
Having just gone through it with absolute certainty my position is that nothing other than WhatsApp was compromised.
Likely the WhatsApp Business is simply
Linked to the same number for 2FA or recovery.
View attachment 1920431
You are right however that there actually is no reliance on any other device. WhatsApp just syncs all the chats directly off server.
The lesson here is to put a PIN and password on your WhatsApp/Telegram/Signal and any other account that uses a phone number directly.
Or switch to an eSIM.