Switching sims doesnt log me out of whatsapp either - as far as i can tell its totally optional whether i want to update my whatsapp number to my phone number or not.

Yes it’s true that switching sims doesn’t log you out, but if you decided to intentionally log out from account A and create a new account B, you would need a second SIM card, that’s what the parent comment was trying to explain.

Makes sense.