Good Afternoon Victor,
I would hazard a guess that if this was done in Tableau the population bin legend was done as a separate sheet and 'floated' on top of the map in a dashboard.
Here is an example that I've done with both the default color key generated by the dashboard (upper left) and a floated view formatted to be the color key similar to what you see there (lower left).
Feel free to download the attached workbook and reverse engineer.
If you found this helpful and/or it answers your question please mark it as such so that others can find it quickly.
Tableau Online Deployment Advisor
Map with Bin Key.twbx 202.6 KB