I have heard a similar thing about mattresses but it was that you should never use a second hand mattress for a baby. Apparently, bacteria can colonise the mattress and when a new baby sleeps on it, it can overwhelm the baby's immune system.
I'm not sure I believe that .S.I.D.S can be solely attributed to fire retardants sprayed on mattresses because babies have been victims of cot death way before those products have ever been used.
I think it's best to just follow the advice of sleeping your baby on his/her back and not letting him/her get too hot.