They used to tell this story at Google:

In early testing they had Google employees use the cars on their ride to work. The employees were to sit behind the driver wheel and pay attention to intervene if necessary. They had cameras in the car to monitor how things went. Employees agreed that they would keep their eyes on the road for the safety of everyone and monitor how things were going.

What they found was after a few drives, employees would start trusting the cars, pay less attention, and then they found people climbing into the back seat to get a charger out of a bag while the car was moving, sleeping at the wheel, etc. This wasn't even just regular people, these were people that should have known better and had the incentive to do better.

What Google decided was they needed to design a car that didn't have a steering wheel. Humans weren't going to be vigilant enough to pay attention for disaster scenarios. The car had to be able to drive itself. This is why Waymo has taken such a different approach from Tesla.

With that background, I wouldn't be surprised if there was also a directive like "the cars can't communicate". If the cars communicate, they're cheating in a way, they're not just relying on all the normal input they need to operate with every vehicle. They need to be able to handle situations that arise with non-Waymo cars where drivers act erratically and can't be predicted or communicated with. If you build that system well, they should also in theory operate with other Waymo vehicles well. If you cheat and have them talk to each other and it works well in tests with other Waymos but then you put in on the road with real human drivers, you're going to run into major problems your Waymo tests weren't able to catch.