This has less to do with skiing open or closed terrain as skiing safe or unsafe terrain. A closed trail is unfit to ski, that can mean a number of things. I've skiied across closed trails in the east in the spring going from glade to glade. To be sure, the trail was unfit to ski top to bottom but no problem to traverse across. In his situation, the trail was closed and clearly there was avy danger and it was a problem to ski it at all.
In my case, I felt confident that I could assess the trail to ski, because the only concern is 'will it ruin the bottom of my skis or not in these spring conditions'. However, when you're in the rockies, in an area with avy danger etc, it's more complicated than 'will i ruin my skis'. If you're an experienced backcountry skier and assess the slope for avy danger, deem it safe, are familiar with the terrain and don't expect exposed rock danger, then skiing the closed trail isn't as reckless or terrible to do, but it's not clear to me that this was the case.
That said, any time you ski a closed trail you are breaking the rules/law, potentially creating dangerous situations for others etc, but you're "an idiot" for skiing in dangerous conditions, not for skiing on a closed trail. In this case, I'm not going to say this guy was an idiot, but he made a bad decision, and that decision was to ski on unsafe terrain. To be sure, Snowmass made a big deal about the trail being closed, in order to waive liability and retain public trust.
It's a subtle and mostly irrelevant point, but there it is.