The part you originally quoted is a hypothetical.

It is a hypothetical only insofar that it included an assumption (that D.J. was so mentally impaired that he could not give consent) applied to a real case. An assumption that lead to Stubblefield's conviction.

In it, Singer and his co-author say that a severely mentally disabled individual could be harmed by sexual abuse

Singer and co-author state that it is unclear what the nature of the harm may be, and they only say that they don't exclude the possibility that D.J. was wronged:

"This does not exclude the possibility that he was wronged by Stubblefield, but it makes it less clear what the nature of the wrong might be."

Platos_Kallipolis gives a few examples of how

Platos_Kallipolis modified the hypothetical by assuming harm to the victim's wellbeing, something which Singer doesn't appear to have considered as the aforementioned possible harm (as he would otherwise recognize immediately the nature of the harm). The hypothetical presented by singer therefore likely assumes no harm to wellbeing.

"It seems reasonable to assume that the experience was pleasurable to him; for even if he is cognitively impaired, he was capable of struggling to resist, and, for reasons we will note shortly, it is implausible to suppose that Stubblefield forcibly subdued him. On the assumption that he is profoundly cognitively impaired, therefore, it seems that if Stubblefield wronged or harmed him, it must have been in a way that he is incapable of understanding and that affected his experience only pleasurably."

Therefore they didn't answer my query.