Technically, he's totally right, as every kind of activity is it's own niche of activity. That niche might be huge (see the Marvel Cinematic Universe for movies, for instance), but it's still a niche. Anything that isn't technically niche doesn't have it's own niche and is doomed to fail. Even a monopoly would technically be a niche - it just fills it up to 100%.
Console gaming is a huge niche worth many billions, so it's rare to be called a niche, but it's one nonetheless, just as is PC gaming, tabletop gaming or mobile gaming.
Niche is a specialist segment of the mainstream the important words for you to think about when deciding if you considering something niche are segment and specialist.