Can customers who order through Talabat join my loyalty programme?

Not from those orders. Stamps are counted against a customer we know by their phone number, and a delivery app hands over a masked number rather than the real one, so an app order creates no customer and earns nothing. Orders on your own page do count. The usual move is to print a code to your ordering page on the receipt so an app customer becomes yours on their second order. The Orders from delivery apps tab in the docs explains why app orders earn no stamps.