I had to make a post on this since it poped up in my feed.
Lets talk about that line first. How is thief, guardian, and mesmer a pet class? Thieves don't even summon any thing without using thieves guild and are you telling me you are losing to thieves USING THIEVES GUILD?
Second, WvW is designed to be unfair. Yes thieves are unfun to fight in WvW due to the ability they have to reset, but that isn't solely because of stealth they are able to just shadowstep and break combat because the WvW in combat breaks to easy due to it not being designed for small scale fights.
Third, thief in general is a harder class to play THE CORRECT WAY. Yes any one can spam three like a lot of thieves do to get kills on low skilled players. So its only natural for it to be more punishing to fight.
Fourth, If you really wanted them to fix the balance in WvW for smaller scale fights there is two simple ways to do that. The first would be to nerf CELE gear in WvW, if cele gear had the same stats as it did in PvP no one would use it. The second thing to do would be to just put in the PvP combat/ make it harder to break combat in WvW. The downsides of that is in large fights it would be a little bit of a problem, and that is what WvW is designed around.
The balance in GW2 has never been prefect to begin with, but there are some outliers that are just straight not fun to fight. I actually do agree that thieves are not fun to fight but I DO NOT think they are overpowered as a class.