I don't think this is saying that the ratings are unreliable, but rather that searching by rating isn't a guarantee that a high-rated restaurant will show up due to the other factors at play.

You don't get a sorted list from highest rated to lowest rated, but rather, momentum of reviews, number of reviews, changes in rating etc.

My suspicion is that there probably is also a noticeable difference between companies that advertise on Google vs. those that don't. Anecdotally, the gym closest to me has higher ratings than all the other gyms in my area, but when I moved to the area it never showed up on Google Maps for me. It was only by walking by it that I decided to look it up on Google Maps specifically by name that it showed up for me.