Yes, it's a crude metric. The other thing is disengagements are self reported, and an unplanned disengagement doesn't necessarily mean a crash was narrowly avoided, it may be an overly cautious test driver, or the result of some non-safety critical intervention.

After the core software problems have been solved, the key challenge shifts to discovering edge cases, which become progressively more marginal the further advanced the software becomes, and the only way to find these edge cases is through 1000s of miles of driving waiting for unexpected situations to show up.