You're not wrong, but it is more complicated than that. Lidar gives you an array of 3D points and intensity, corresponding to where the lasers bounced back, and how strong the reflection was. Roughly speaking, from there you have to decide 1) which sets of points belong to the same object, 2) what those objects are, and 3) what those objects intend to do in the future.

So yes, a lidar-equipped AV completely not sensing a pedestrian would be surprising, but you can see how it might have incorrectly classified the pedestrian, or misunderstood the pedestrian's intent.