Again, we’re talking about different linguistic issues, which I’ll demonstrate below. I see now that my example wasn’t a good example because it conflates a consequence of the problem with the problem itself.
The ambiguity
There are two different ambiguities. You’re talking about ambiguity over the subject whereas I’m talking about ambiguity over the sense of the pronoun.
“I was with Dan (they/them) the other day. They hadn’t brought a poster they needed and went back to the car to get it.”
No ambiguity over sense of “they”. No ambiguity over subject.
“I was with the newlyweds the other day. They hadn’t brought a poster they needed and went back to the car to get it.”
No ambiguity over sense of “they”. No ambiguity over subject.
“I was with Dan (he/him) and Steve (he/him) the other day. He hadn’t brought a poster he needed and went back to the car to get it.”
No ambiguity over sense of “he”. Ambiguity over subject.
“I was with Dan (they/them) and Steve the other day. They hadn’t brought a poster they needed and went back to the car to get it.”
Ambiguity over sense of “they”. Ambiguity over subject.
The ambiguity over the sense of the pronoun is the confusion. That’s the problem. The ambiguity over the subject is a problem but not the problem I meant.