According to the article, channels are slow because they use mutexes under the hood. So it doesn't follow that channels are better than mutexes for large N. Or is the article wrong? Or my reasoning?
I have replied to another comment with more details: the channel mutex is not the same one that sync.Mutex is using.
The article that the OP article references does not show the code for their benchmark, but I must assume it's not using a large number of goroutines.