According to the article the car was traveling at 17 miles an hour before it began braking. Presumably this was in a 25 mph school zone, so it seems the Waymo was already doing exactly what you describe - slowing down preemptively.
A 25mph school zone? That seems fast. 15mph would be more the norm, which is in line with the 17mph the car believed itself to be traveling.
FYI, unless you are a commerical truck, a cop, or a racer, your speedometer will read slightly fast, sometimes as much as 5 to 10%. This is normal practice for cars as it limits manufacturer liability. You can check this using independant gps, ie not an in-dash unit. (Just imagine the court cases if a speedo read slower than the actual speed and you can understand why this started.)
This is close to a particular peeve I have. Occasionally I see signs on the street that say "Slow Down". I'm not talking about the electronic ones connected to radar detectors. Just metal and paint.
Here's my problem. If you follow the instructions on the sign, it still says to slow down. There's no threshold for slow enough. No matter how slow you're going, the sign says "Slow Down". So once you become ensnared in the visual cone of this sign, you'll be forced to sit stationary for all eternity.
But maybe there's a loop-hole. It doesn't say how fast you must decelerate. So if you come into the zone going fast enough, and decelerate slowly enough, you can make it past the sign with some remaining non-zero momentum.
You know, I've never been diagnosed on the spectrum, but I have some of the tendencies. lol.