I don't quite get this... According to the bar graph, the automated systems fail to correct around half of even orthographic (spelling) mistakes. One example is

I don't quite get this... According to the bar graph, the automated systems fail to correct around half of even orthographic (spelling) mistakes. One example is "advertissment", which macOS is now trying really hard to correct against my will in this text area. Another example is "From this scope, social media has shorten our distance", where "scope" is supposed to be "perspective". That seems to be something that machine learning should easily pick up on, and indeed, when I just tried it on google translate, I couldn't get it to make this mistake without my original (german) sentence also encroaching awkwardness. So I'm unsure how much value it is to win against systems that fail rather spectacularly. I also don't quite understand why you would need manually-created data for this task, instead of just buying everything ever written in TOEFL essay questions and pitting it against the New York Time's archive. It's obviously quite likely that there are good reasons for all this. They may have thought a bit longer about it than I just did.