Nice post. I think there are a large amount of factors that run contrary to the popular opinion that playtesters are getting an unfair advantage.
For context, I am a reasonably competitive AoS player who has played against England multiple times at the 6N and thus have a vested interest in all of their players being permanently banned from everything.
1) Wrt to singles events - playtesters are already picked because they've already proven themselves to be among the best players. If you're winning/nearly winning all your events - there isn't much room for an unfair advantage anyway.
2) As discussed, new books are not playtested against the current meta - see how OBR's standing changed after Tzeentch was released.
3) The top lists and strategies for new books are very often discovered by the community before the book is even released, I can't think of a single example of a playtester breaking out a new-book-list that I hadn't seen multiple times before in a WhatsApp group.
4) At the last 6N, Slaanesh had just received a new book shortly before list submission. As I'm sure people are aware that Slaanesh book was one of the most powerful books AoS has ever seen. Despite this, the England team did not have a Slaanesh list in their roster, suggesting that despite insider knowledge they had not had sufficient time to prepare a Slaanesh build for the current competitive team meta.
5) Even if we were claiming that a playtester advantage existed, we have to think of a reasonable response. It is good for the scene to have top competitive players playtesting new books, so we have to weigh up whether the advantage these players (who are already winning basically all their games anyway) would be large enough to merit a decrease in the quality of playtesting. An extreme example would be if we banned playtesters from events, we would most likely find that competitive players would give up playtesting - and playtesters would be players who didn't go to events.
6) Any advantage of playtesting a book decreases as time goes on, the longer the book is out the more it gets "solved" by the community. If we really are worried about playtester advantage, we can set up deadlines for list submission so that there aren't any last minute army releases to which only the playtesting team could react.
Those are my main thoughts, I think overall the most compelling evidence is from the OP - there just isn't data to support the idea that playtesters are gaining any advantage at actual AoS events.
Much love,
Jp