That site has a count of sites moved off WP Engine (currently 17K) and a graph showing the total rising over time. And some JS highlighting individual sites. So it obviously is recording sites that moved off WP Engine.

It looks like they have a list of ones not moved from them too... that doesn't change the fact they are recording the ones that moved. Either list... how is it 'anticompetitive'? If they moved, they moved; if they didn't, they didn't.