The claim isn't that your saying no to fulfilling orders,
  it's that your saying no to giving them a discount.

Well, no. This is the comment I responded to:

  Unless something has changed over the last couple years,
  restaurants opt in to being available on those apps. 

That very much asserts that the issue is about accepting orders. Doordash et al were initially opt-out.

Yeah, like the restaurant can say “no” to giving a discount, they can say “no” to people wanting their food to get delivered now. It’s just that now it’ll be a bad business decision probably.

Everything is possible. And every choice has its own set of tradeoffs. But no, there’s no time machine to the pre-Doordash world now.