Your original comment said it was wrong to exploit a bug, to which the parent poster retorted that this was a feature and not a bug.
Here, you've gone further to claim that this behaviour is stealing, and I'd like to explore that for a minute: what possible moral or legal right does Doordash have to an operating profit when it deliberately operates at a loss?
By all accounts, this below-cost pricing is predatory behaviour on Doordash's part, not the customer's: they seem to break into a market by offering delivery at a subsidized rate, then they take data based on those rates and try to strike fee arrangements with restaurants. At first glance, it seems like they sell themselves based on inflated numbers from the discount period, without disclosing that they were in fact offering customers a discount.
I see no ethical fault in beating a (sophisticated!) predator at their own game, but where do you reach the alternative conclusion?