Singular they

[Andres Fontalvo]

Hi,
I submitted my first review, where I wrote:
"The author is unapologetic in their portrayal of Shanghai."
To my understanding, this is valid use of the singular they, but it was marked as an error by an editor. The editor suggest I use either his or her. However, the guidelines say:
" Do not specify the author's gender or use gendered pronouns unless you are certain of the author's gender. When in doubt, use gender neutral terms (e.g. "The author does a great job at..." instead of "She did a great job at...)."
I believe the author is probably a "he", but since I couldn't find pronoun specifications on their website, it is impossible to be certain.

So, is the sentence correct, or should I completely reword it?

[Shirley Ann Riddern Labzentis]

You are right on this one. You are not supposed to use the author's gender unless you are 100% positive of the gender.

[Alys Sterk]

Yes, you are right, unless you can tell from the author's biography that they identify as either a male or a female.