Uber and Waymo are fundamentally different businesses.
Waymo has to buy each of its cars, so it can only scale as quickly as it's willing to invest capital.
Uber doesn't own the cars, it just has the rider + driver network, which can grow without limit in theory without Uber having to put in more capital.
This is another example of atoms and bits behaving very differently.
This means that Zoox and Waymo, assuming they both have a similar product that is above the quality bar for most people, will primarily compete based on how much capital they want to put into supply in a given market.
Waymo can flood any given market with way more supply, making it hard for Zoox to get a demand toehold.
The best bet for Zoox would be to go into markets too small for Waymo to bother with at first, and then hope to establish a toehold network of demand, and then hope to hold on to it with a better brand.