Many a student fail because of their poor attendence.

[Rover_KE]

I suppose "themselves" was used as a gender-neutral singular reflexive pronoun.

I'd have said '... who derides themself'.

Themself usage note

The standard reflexive form corresponding to they and them is themselves, as in they can do it themselves. The singular form themself, first recorded in the 14th century, has re-emerged in recent years corresponding to the singular gender-neutral use of they, as in this is the first step in helping someone to help themself. The form is not widely accepted in standard English, however.

(Oxford)

 

[Rollercoaster1]

I think of another example where usage overrides logic: "More than one person was injured".
"More than one" denotes at least two, but it takes a singular noun and verb.
We learners have to toe the line.

Is it wrong to say 'More than one person were injured'?

 

[emsr2d2]

Is it wrong to say 'More than one person were injured'?

Yes.