I've found the answer for myself by playing around a bit more. It seems that in selecting the "support angle", a higher angle means _more_ supports. This was confusing me, because I had assumed "support angle" meant the maximum angle of overhang before a support would be generated. I guess it does actually mean that, but it counts 90º rather than 0º as vertical. In any case, supports are turned off simply by changing it to 0, in which case the text "not recommended" does indeed appear.