Further to Jaba's point and quote, although I have been known to refer to a paper checklist in the aircraft, I've yet to see anyone use one for the external portion of the preflight checks. How adding more verbiage to the POH would assist in preventing this sort of error I don't know.
I'm not sure that I'm quite ready to hang the ATSB from the nearest tree over the speedbrake issue though, I very much suspect they are quoting either the maker's bumf or the pilot. It is a side issue and probably didn't get a great deal of attention once it was determined that they didn't cause the unhappy event.