Why did a customer get a WhatsApp from us at six in the evening about tomorrow's order?

That is the day before reminder for an order booked for later. When an order is booked at least a day ahead, a reminder goes at 18:00 the day before, in your restaurant's time zone. It is not sent if the order was cancelled first, and orders booked for the same day never get one. It reads as a helpful nudge rather than marketing, and it goes out on your own WhatsApp number. The Orders & delivery tab in the docs explains the reminder.