Firstly, Singed is in no way a tank. He lacks any form of initiation ability and his only form of crowd control simply chucks a champion slightly behind him but in no way disables the target.
He is however a perfect example of a Tanky dps, he survives very long and deals noteworthy damage during that period of time. Difficult to kill with moderate damage but lacks initiation skills and CC = tanky dps.
That said, being in the category of a tanky dps I do agree he has more survive-ability than he ought to. However he also sports the lowest immediate damage capabilities of all the tanky dps champions -he has the highest long term damage capability (should noobs chase him).
Generally I see Singed's buy a RoA first... This should be the first indicator that he is not a tank -you don't see Rammus buying a RoA simply because his powerball and ult have AP ratios.
Also, downvoting doesn't make me wrong -just ask a Riot employee. Or just watch his spotlight, clearly states that he's a tanky dps.
His slow is quite potent and his toss is amazing as well. He buys RoA first because his passive gives him health from mana (obviously). RoA is a different item for him than it is for Rammus.
He is among the toughest champs in the game, and his main utility is not from his damage but from his ability to dive straight into the enemy team and live, the hallmark of a true tank.