That link is about practice in the Euro (currency) zone, I've not read the document yet, however from a UK background the key point is the following (and from a very fast skim read, it looks like other countries share this perspective):

Legal tender is something someone is obliged to accept to settle a debt. I.e. where a debt exists already, it can be settled.

It does NOT confer onto you the right to make a purchase using that method of payment. The person selling is free to refuse business unless their chosen payment method(s) are used - thereby preventing a debt situation from occuring.

Hence, an airline might have T&C which say they have not agreed to transport you, until you have paid. So no debt obligation will exist at the time.
Similarly, they have not agreed to transport your overweight luggage until you have paid, so no debt exists.