I think people mean well but it may come off as wrong. Maybe your friend just wanted to help. I know it is annoying but sometimes things just come out. It's like when my sister has a headache and I ask her if she tok a tylenol for it. Well duh! It's not like she is an idiot and never had a headache before.It is just a reaction.
As for parents of onlies- No- I do not think that you are terrible parents without a clue. I do however believe that you don't know what it is like to have many children since you don't have many children. Having a sibling is not the same thing. It is not meant to be an insult. It's like me trying to give parenting advice about teens. My kids aren't teens yet so how could I possibly know what they are really like? KWIM?
I agree with the bolded part. Just from my experience going from one child to two, it was a huge change. I don't think a parent with an only child is any less of a parent than someone with six kids but I do think it's a different experience and a different dynamic.
It is just a reaction.
I just considered the source. Then she had a baby.
Sometimes telling her about deadlines is like talking to a brick wall. 
Well heck, I have an older brother so I know what it was like with the two of us.