[70forty]

"themself" follows the same pattern as "himself", "herself", "ourself", etc. As a commenter pointed out, "them" is frequently used as a singular object pronoun:

If a student has an inappropriate question, whatever you do, do not berate them.

corpus.byu.edu/coca/ returns 95,000 hits for "themselves" vs only 43 for "theirselves".

[gr3yh47]

This is, however, formally incorrect.

"If a student has an inappropriate question, whatever you do, do not berate him."

would be formally correct

edit: and your corpus search correctly shows that the plural 'themselves' is correct over 'theirselves' while having nothing to do with the discussion at hand about the singular

[Symmetry]

What are you consulting to find that the correct replacement for 'them' in formal writing is 'him' here? There isn't any College of the English Language to make authoritative prescriptive statements, and the Chicago Manual of Style at least has been against the use of 'him' here for several editions.

[gr3yh47]

I am not claiming that 'him' is a replacement for 'them'

'them' is plural and flatly incorrect when used as a singular pronoun.

edit: Again, 'them' is formally incorrect but informally acceptable in the singular usage.

[70forty]

'them' is plural and flatly incorrect when used as a singular pronoun.

Why? How do you know this?