It is certainly not open source, but it is a copy left license.
You could make the exact same argument for literally any open source project where someone forks it.
The "power" of open source is that in theory if the creator went rogue that the public can switch to the fork as the main app used.
I really fail to see if the dataset is open, how the data itself (of course not the web domain or front end) would be any different in a fully open source project. Maybe the transfer to a new app in the last case would be quicker, but that is about it. The dataset is the gold here.