I have not found any mention of Bancontact in the official FAQ.
But I have noticed it in the Kassa Compleet Portal interface.
I have got an answer from the Kassa Compleet support:
Please know that Kassa Compleet accepts payments with Bancontact. Please find all payment methods listed on our website: https://www.ing.nl/zakelijk/betalen/geld-ontvangen/kassa-compleet/index.html