The honest argument is that we have seen Tesla’s technology exposed to unsupervised and uncontrolled adversarial conditions across hundreds of wildly diverse cities in the USA, whereas we haven’t seen Waymo vehicles doing anything outside of curated geofenced areas or curated marketing videos. Right now if you dropped a Waymo and a Tesla on an unsealed road in Michigan, one of them will drive at least as well as a human learner driver and the other will probably refuse to drive.
I agree that Waymo could well be far ahead of Tesla, but there isn't enough information in the public domain to say this with confidence. We don't have the ability to make a proper comparative assessment.