Lets put it this way. In a vacuum, "he is coming" refers to a singular male. What information does "they are coming" or "they is(?) coming" provide? A group of people on unknown gender or a singular person of unknown gender?

The use of they when referring to a singular only works when there are previous information or referring to a singular in a group "each of the group tells me they are coming"

The use of they to refer to specific known singular gender neutral person is new. It might be politically correct, but it creates confusion.

There is nothing wrong in my opinion to omit pronouns when referring to a specific known person. "autumn is coming" refers to a singular autumn and is gender less.