Well I guess they dont really have any concrete evidence do they?...
Although they say there was some evidence to show an increases risk of SIDS with bubs on second hand mattresses, they actually do not even know what condition these mattresses were in, or what position the babies were sleeping in on them... so its hard to really call it proper 'evidence' isnt it.

So I would say that a well kept & clean mattress, especially if you have been using a mattress protector, should be ok....