Waymo needs $16B to build what Tesla already has: manufacturing capacity. Without that, there are only so many cars they can put on the road. They've proven they can do the rides. But they haven't proven they can do it cost effectively. To scale up and start making a profit, they'll need to start building/buying lots of Waymo cars. That's not going to be cheap or fast. That's going to involve a lot of capital expenses.
Tesla is the other way around. They can definitely make lots of cars and make a profit. But they haven't quite gotten FSD to the stage where it can do rides properly. Supposing they at some point figure that one out, they are very well positioned to start producing vehicles by the hundreds of thousands pretty soon after. That's indeed the premise for their valuation. It's risky but not completely without merit.
Another point to make is that Waymo and Tesla are not going to have this market to themselves for very long. There are quite a few autonomous ride hailing companies serving rides at this point. And while the attention is often on the US, China is moving pretty quickly as well. Several companies competing there in several huge Chinese cities, for example.
On the US side, I think there are a few players that might become competitive soon. Zoox is looking pretty solid. And Rivian is rumored to be pushing autonomy as well. There are a few more players in various stages of technical readiness.
The real battle will be in a few years when we are past the basic "does it work", "is it safe" questions and legal approvals all over the world become more routine. Then it will be all about volume and scaling. That's going to take probably at least until 2030.