That's not precisely what the report says.
It says that a) if you suspect differential treatment you can't localise it to where in the delivery chain it occurred with the current tools [hence not "fit-for-ofcom's needs" - who do you chase after and how?]; b) there are lots of differential treatments that can't be detected with the current tools; c) The differential treatment is the wrong question - you can test for a quality floor much more easily.
All those scientists who look for "signals" in randomness have models for the signal they are looking for, there isn't a model to test against at present (and "net neutrality" is an aspiration not a model)
disclosure: I was one of the reports authors.