I’m downvoted every time I point out this very simple thing we can do to be more inclusive.
I don’t see any indication that the author isn’t a native English speaker and I wouldn’t want to make any assumptions in that regard either.
But if the author very clearly isn't used to writing in English in the first place (you cannot possibly have missed that) then you can start gently with the fact that we don't use genders for all nouns, as they may not even be aware of that, before you go in with the big guns of calling people's efforts at writing in a foreign language 'nasty'.
If I asked you to write a blog post in Sumerian and you made a faux pas and I jumped all over it you'd be a bit put out wouldn't you?
‘The singular “they” is a generic third-person singular pronoun in English. Use of the singular “they” is endorsed as part of APA Style because it is inclusive of all people and helps writers avoid making assumptions about gender. Although usage of the singular “they” was once discouraged in academic writing, many advocacy groups and publishers have accepted and endorsed it, including Merriam-Webster’s Dictionary.
‘Singular “they” is covered in Section 4.18 of the APA Publication Manual, Seventh Edition’
https://apastyle.apa.org/style-grammar-guidelines/grammar/si...
I'm honestly always a little surprised when this topic comes up with the number of people who find this grammatically jarring. To be fair, I am fairly young, but I do remember learning about singular "they" while I was in primary school in the early 2000s. Is the issue that this isn't taught in US schools, hence all the confusion?
In much the same way, it's not wrong to use plural "they," but it is no more correct than singular "they." Language is descriptivist. Its rules are determined by usage and clarity as well as historical usage patterns, all of which are subject to change. As of of release of the APA Publication Manual Seventh Edition in October 2019, the APA acknowledged such language usage pattern changes and by doing so, the major standards body which codifies style for major publications has adopted singular "they." Resisting such change is fine, but seems a strange hill to die on.
https://www.python.org/doc/sunset-python-2/
https://en.wikipedia.org/wiki/APA_style#Seventh_edition_of_t...