Coinbase almost definitely acts as a market maker as well. I would be very surprised if they didn’t at least seed the order book a little before listing new pairs.
Further, Coinbase often lists some new assets on Coinbase earn where it gives them for free to users.
That's not how coinbase operates. You have a "Buy <Coin>" button, and you expect to get that coin without fail, every time. If a new token launches, you should be able to buy it without waiting for users to deposit and sell. Maybe you're confusing with coinbase pro, which uses the more traditional order book model, or with other exchanges?
Coinbase adds new assets to the pro option first, establishes a market, and then adds to the retail Coinbase "buy button" app. When a user hits the retail Buy Coin button, the internal market maker fills the order against the coinbase pro's order books.
Do you have evidence that Coinbase doesn't provide liquidity for it's tokens? It's a pretty safe assumption.
crypto exchanges are all over the place regarding this