Yeah, I'm not enthralled with Guardians (at all) but I don't see how you can call it short, at least for a roller coaster. All coasters are short if you're comparing them to elaborate dark rides. Big Thunder Mountain Railroad is significantly longer, but that aside, I don't think any of the other WDW coasters are longer than Guardians (at least not by more than a few seconds).
The only thing I can think of is the fact that it replaced one of the longest (or maybe the longest?) theme park experiences around, and that they talked about it as though it was going to be more than just a coaster, so maybe people are grading it on an unfair curve?