That could easily be part of their prompt. I just did a quick test telling Copilot to "add a few spelling and grammar mistakes to look like a human wrote it" and it does a reasonably convincing job.
The errors are more than superficial; few authors would be willing to have AI screw up their work enough to invert the meaning of some of their sentences. https://news.ycombinator.com/item?id=47169256