It's a single data point / anecdote. These investigations are incredibly thorough, precise, and authoritative so it makes sense they'd seek to exclude that kind of information until they knew for sure.
An irrelevant one in this case. This report needs to find out the technical/training/... reason, not why the other pilot needed a ride. I'm sure they'd have included it if they wanted to recommend having three pilots on every flight.
And a very relevant one, in this case: trim runaway with pilots not understanding it's a runaway. The visiting pilot does recognise the situation and restores expected flight dynamics. What went wrong, why did not two pilots realise something that is traditionally a known case and well understood? Because it didn't look like a traditional case.
I'm not sure that thorough and precise implies they would seek to leave out things. In fact, one would expect the opposite.
One data point may lead you to bad conclusions, like in this case "one third of pilots don't know how to disengage the MCAS" -- the fact is we don't know what ratio. It might be higher, it might be lower, but one data point isn't valuable or thorough. In the preliminary report they mentioned someone else disabled it. In their final, I'm sure we'll get the details.