Not entirely true. Your HMG will do shit in front of the ZIS guns in my previous example. You can use infantry neither to kill the ZIS guns bercause the ISU behind will erase it. So if you know what you are doing you will slowly push the Elefant with its entire suite to the edge of the map.
An Elefant can deny all armor but an ISU can deny everything. You still can cap in front of an Elefant, you cannot cap in front of an ISU without loosing all infantry that is trying to do that. Capping will win you the game eventualy, not killing the opponent's tanks. Can you put aside your fanboyism a little and see the difference? The other reasons were stated by many before so I won't count them again.
That is the reason for which I am sure ISU is OP but it's quite debatable if Elefant is.
All that doesn't matter since Ele will 3 shoot ISU from across the map. After 2 shoots ISU usually retreats.