I didn’t know they are instant cast (were is more precise; Blizzard removed them for the moment) and are used in a normal high-end rogue rotation. That’s a pretty cool way to get more dps. I wonder if they generate personal threat and could be used by tanks.
MMO Champion’s commentary is particularly good, showing that the grenades truly are a normal part of how their rogue did his job.
The question is whether it was obvious to Ensidia that something was askew. This encounter was not on the PTR, so they wouldn’t have known what it was supposed to be like. They didn’t have a comparison. Sure, these platforms rebuilding might appear like an unintended mechanic, but when you only have limited attempts and have never seen it before… perhaps it looked like a deliberate encounter feature. They would have had no idea the grenades were causing it.
However, that’s the question: is there something we don’t know, that Blizzard do, about Ensidia having some way to know without any doubt they were exploiting the encounter. I think they are innocent, very clever cheats, and I’m curious to see how it pans out.
Update: the logic is too compelling, they knew it was an exploit. Also see Psy’s comments below on the access world top guilds have to Blizzard.