Sure they would, they'd just be smarter about it. The point of hacktivism is publicity. If no one knows what you're doing, you're not being particularly effective.
A lot of the content that surrounded Stuxnet also hints to further organizations existing behind the veil. There were at a minimum 3 0-day exploits present in the virus that would have to have been operated from behind the scenes. It is extremely unlikely that a single group was able to create such a virus without external resources.
In the end you don't need to issue press releases and the like. You need to get in, do your damage and get out. Let the damage reveal itself in time and its considered massively successful. Those fighting the Iran nuclear program did more while keeping their mouths shut than any loud group ever has.