1: If someone condescends to do something, they agree to do it, but in a way which shows that they think they are better than other people and should not have to do it.

[Collins COBUILD Dictionary]

I always thought of someone as singular gender-neutral pronoun. But in the above definition, they is plural while someone is singular, though the referent of the both is same.
I think one/someone/they are quite impersonal here, could be seen as both singular and plural.
Hi Jackson

The word 'they' is often used instead of having to use the more awkward-sounding "he or she" or "he/she" or "that someone".