> My reading of this is that the threshhold for the USA government to withdraw financial (and other?) support for disabled people is too low, is that right?

In general, step functions (earn 200 get benefits, 201 no benefits) are really really bad from a public policy perspective. If you want to get people out of these dependency traps then you need to taper them more effectively.

Even tapering them is problematic as you end up with astronomically high effective marginal tax rates, but it's better than a hard cut off.