AIUI, that $1.90 (wasn't it $2? no matter) wasn't intended to be a particularly good number, but was picked because having something was necessary, a lot of desperately poor people had less so it was in the right range, and no other number seemed better at describing desperate poverty. It continues to be used because it permits comparison with older data. I haven't noticed anything that says it's bad — for example, I haven't read that there's a lot of people whose income increases past $1.90 but the upward movement stops shortly above it, so that using $2.50 or $3 would lead to very different graphs.

The article seems to imply that using a much higher number would be substantively different, but I can't really tell why. Is it really just saying that counting desperately poor people is bad, and instead the number of poor people should be counted? Or is it saying that helping most of the desperately poor become merely poor is racist, we rich people would do more for them and faster if they had the Right skin colour?