Never heard that dresses were gender-neutral for small children in the 19th century. Seems I've seen plenty of old photographs that show boys wearing traditionally "masculine" clothing, though commonly with short pants, transitioning to long pants during adolescence.
It's hard to say if a 5 year old is a boy or girl withought clothing identifiers so it's not always obvious. However, from a practical standpoint a dress takes less effort to make, you don't outgrow them as fast, and it makes changing faster so I can see why it would be the gender neutral choice.
I can't wait for the day people just don't give a crap if the 5 year old has a dick or not.