I wonder where they're getting the data. I assumed OSM but I found at least one place on this map that I couldn't find on OSM or in All the Places (but it is on Google Maps).
If anything it seems to err on the side of false positives. A lot of these places seem to be traditional gyms or trampoline parks that I don't think have any bouldering walls.
I used Google map API to fetch all the gyms then cached it locally. It only allows (at least from what I can find) text search within a viewport. I think there are some problems when loading boulder gyms in France. At first glance, I thought France was not really into bouldering...
If anything it seems to err on the side of false positives. A lot of these places seem to be traditional gyms or trampoline parks that I don't think have any bouldering walls.