The recommendation letter thing appears to be a misconception. My understanding is that it's been conflated from the internal review letters that were written. There was also at one point an allegation in a lawsuit from one of the students he later abused saying that the Blackhawks provided a positive reference, but I believe that was later amended to only allege that they verified his employment.
HOWEVER, I think the sentiment is correct anyway. Even without a recommendation letter, simply letting him walk away quietly obviously amounts to an implicit reference. Worse than quietly, someone submitted his name to be engraved on the cup and I don't think why or how that happened has been adequately answered. What better reference could you ask for? Stan Bowman and the Blackhawks' actions directly resulted in him having access to minor victims.