Noun sense 4.4 there comes from the medical use, which is a euphemistic reference to verb sense 1.1.

I don't know whether the noun retard developed within medicine from the medical use. But this much is clear:

1. The word retarded (not retard) was employed in an effort to be technical and sensitive in referring to people with mental deficiencies;

2. The same word, retarded, entered general use in reference to people with (more broadly-construed) mental deficiencies;

3. The noun retard derived straightforwardly from retarded, in the sense "person who is retarded". This might have happened before step 2 and then entered general use in parallel with retarded, or it might have happened after step 2. Doesn't really matter.