word choice - Why "themselves" instead of "himself" when referring to third-person singular?

I've read today a comment from a UK user that sounded weird to me:

so the OP is shooting themselves in the foot here with the tone then.

I would have said instead:

so the OP is shooting himself in the foot here with the tone then.

I've taken a look at this but it's way out of my league, and appearently doesn't even answer this question.

Why did he use the third person plural while referring to a third person singular ?

Answer

In English, "they" is used as a singular personal pronoun when the gender of the subject is not known.

As gender politics have evolved over the past half-century, and the pace of that change has accelerated in the last decade, personal pronouns have proven fraught with risk and created traps for the unwary.

In response to this, commentors have increasingly started using the non-committal -- and more importantly, safe -- singular they to refer to people whose gender is not known with absolute certainty.

Please note that I'm using the word "gender", not "sex", here quite advisedly; sex is biological, gender is psychological (or, in another school of thought, sociological). This (rather new) distinction is yet another driver behind the recent popularity of singular they: even if a person's sex is quite evident (or you believe it is), their gender may not be (e.g. a person who looks like a man may nevertheless wish to be identified as a woman), and using the wrong personal pronoun can land you in a lot of hot water.

In your particular situation, circumstances which may have contributed to the commentor's circumspection are that your avatar depicts both a man and a young girl, and while your name is quite masculine in your homeland (and its own history as you so perspicuously pointed out), in the US, names ending in -a are considered feminine, and "Andrea" is reserved for naming girls.