Makes sense since a window bows out like a dome, so hitting in the center might not break it.
I've seen various YouTube videos of guys tiring their arms out trying to break a car window using Maglites, a hammer, even cops with batons, so that side glass can be tough to smash with something blunt.
That said, as pre-tensioned safety glass, if you can apply force in a concentrated spot, a side window should shatter and crumble fairly easily. That's why those automotive "escape" tools have things like a spring loaded center punch or a Lifehammer with that small but pointed cone to whack a side window to escape a car.
If it's pointed enough, you apparently don't even need to really whack the glass. Even pressing the point against the glass with some force should be enough to shatter it. I saw that on a TV segment about escaping a car that plunges into water. They just used a hardware store center punch tool and pushed it against the side window while submerged and out they went!