Does mobile refuelling with a debit card only work with the Payconiq by Bancontact app?

No. You can use any banking app that supports "Deferred Sales". At the moment, the Belfius app is the only one in Belgium that supports this. If you have both Payconiq by Bancontact and a banking application on your phone, you will always have the choice via a pop-up when you make your payment.