Does Welcome Decks 2016 contain cards that aren't reprints?
There do not appear to be any cards that were only printed in the Welcome Decks.

The most likely explanation I see is that the rares you mention appear in the Welcome Decks but not in the concurrently released Shadows over Innistrad block. So, the quotation you reference could mean


  Each deck includes 1 special rare that cannot be found in other Magic products [that we are currently publishing/distributing]!