0.01 is always less than the minimum.
It turns out it doesn't matter. Just like the "proportion of extreme planets" field, it appears that "proportion of precursor worlds" field has no effect, at least not in the map editor (I didn't bother checking the actual game, as they seem to correspond when the setting are the same). So, your original statement turns out to be correct, and the answer to my question is that, yes indeed, that field is non-functional!
As always, talking to you about planet modding ends up with valuable information. Thank you!