I guess in theory you're correct about (b) but the implications for (a) are that the hourly rate is even lower for people who work few hours. A person who worked 1 hour a week would need to pay the company in order to true-up for the benefits.

DoorDash isn't avoiding anything by not paying benefits -- they are paying cash instead of benefits. Maybe it's the case that the employees should net out more than they do; but that's quite separate from whether they get benefits or not, because it's all cash out the door at the end of the day.