The classes do scale as you mostly would expect them too.
This does not mean “Scale relatively to their position in a tier ranking list”.
It means that the class scales in the same way, that if a piece of gear was good for you back then, the same piece should be just as good.
Ret wasn’t that powerful in Ulduar dps wise. They wanted to stay true to that and succeeded, the buff did not make them too powerful.
Ret was powerful in ICC, and they will be today. Their statement has everything to do with how character’s scale with themselves, and nothing to do with how they scale relative to other classes.
This is what you are taking out of context. Aggrend is clearly referring to Phase 2 and Phase 2 only.