Yeah I don't get this. Just because there are popular alternatives doesn't mean there aren't tons of people for whom Grunt still works perfectly well.
I'm also curious. I don't use grunt because my daily workflow is almost entirely in Visual Studio, but I though grunt was the go-to for automation tasks.
Maybe the author meant that there is a lean towards gulp over grunt? Not sure. but my company still uses grunt in production
There are few JS builders these days - Broccoli, Gulp, Brunch, Cake, Gobble...