"it hits a bit of a raw nerve to be told that their behaviour is worse than other children."
Have to disagree tbh.
Yes, I have an only child. Yes, I would have liked another child ideally but I am also very happy with the one I do have.
But no, it doesn't hit a nerve - because ime what the OP is stating is wholey not true.
I have an issue with a generalisation put about about a group of children that is untrue.
How about:
"Children with siblings are lousy at playing nicely because they always fight with one another and squabble over the most ridiculous trivial things."
Would people agree with that as a generalisation, just because I know 3 or 4 children with sibings who are like this a lot of the time? No, of course they wouldn't! because for the vast majority of children with siblings it would be untrue. Just like the generalisation that only children don't know how to share.
It isn't just through sublings that children learn to share after all. Most 6y have been to school and been engaging with other children for a couple of years at lest, if not years of nursery expereicnes too. Or playgroups, or cousins, etc.