It is important to note that in China, developmental delays are treated with ridicule for the affected and as a source of shame for the families.
I'm not passing judgment on the parents, I'm just pointing out that how society treats developmental delays is extremely important to the quality of life of these people.
Here in the US autism was considered a "mental illness" until the 1980's.
I think this is an important nuance that may be missed in the article. The teacher pulling the parent aside to ask her to check her child made me remember what my partner said about the time they moved to China as a kid and how they were treated in school because they were falling behind their peers. (In that case because they couldn’t keep up with the natives due to language skills.)
I too do not mean this particular teacher is to blame rather than the way society works.