"incentives", i.e. discounts/coupons for passengers or pay bumps for drivers, are a huge part of those marketing costs. So while they're rolled under the marketing budget they fundamentally are a distortion of the price they need to pay drivers or amount they need to charge passengers to become profitable.

A lot of these "marketing" costs are clever ways of hiding costs of providing rides as separate line items.

Yes, but if they were all driver incentives, there would be no revenue according to the model they're using. Customer incentives, on the other hand, are hidden in operating costs and not placed in the cost of revenue where they probably should be.

I was in an empty pool ride for 45 minutes taking me from inner Boston to the suburbs and I paid $5. Unless I'm wrong, that's below minimum wage so someone somewhere is subsidizing the difference