It comes from here:
https://www.fangraphs.com/prospects...t/scouting-pitching?sort=-1,1&type=3&team=atl
Those RPM values are gathered second hand from scouts Kiley and Eric talk to. That 1700 value for Anderson’s breaking ball would be the worst of any MLB pitcher.
That RPM value could be erroneous, but if it is accurate it is physically impossible for that spin rate to produce a MLB breaking ball.
I find this data very suspicious. A guy that throws in the mid 90's and, by all accounts I've read, gets good reports on his curve should have much higher spin rates.