The figure PBL quoted was observed deceleration, i. e. the deceleration that was actually achieved in this landing (derived from the ground speed plot, and supported by the longitudinal acceleration pliot).
This is independent of how it was achieved, and under what conditions.
What is, is.
What was: The plane had a given deceleration
What we don't know whether it was or not as yet: The runway was (very) slippery
PBL stated that one can get a given deceleration
in a slippery runway. I supposed he was basing his statement on the TAM's
PR-MBK FDR.
Then I finished by stating that his statement might not be accurate not because the plane didn't decelerate at that given rate (What was) but because we don't know in what extent that runway was slippery (what we don't know whether it was or not).
If you expect poor braking action, you must brake as soon as possible to get the most out of the available distance. Not delay it.
No controversy about that when you don't have a (very) slippery runway (or don't
think you have).
However, I found this article
http://www.jet-jobs.com/articles/hydropln.html
which I am not certain whether it is accurate or not. It deals with a "Minimum Hydroplanning Speed" i.e., a threshold above which you should not apply braking when facing a slippery runway.
Again, I don't know whether the article is accurate or not.