It's all about what you're nostalgic for. Ennis is 7 years older than me, and grew up reading a lot of the same comics that were popular in Ireland and the UK when I was a kid. He was reading 2000AD, Eagle and Battle, and so has fond feelings for Judge Dredd, Dan Dare and Charley's War the same way a USian fan in their late 40s would have for late 70s-early 80s superhero comics.
At the same time, Ennis' disdain for superhero books comes from the same source - he grew up reading SF and war stories, and would have come to the superhero idea later in life so it would feel sillier. And yeah, you can convincingly argue that Judge Dredd is a superhero (such as Claremont did in Excalibur), at least there's some window-dressing to justify the Judges in a way that superheroes don't have,