This would only check the requirements when the actual entrance of the instance is used.
If "The key requirements were not included on instances, instead heroic mode is available to all players at 80." can be read as, "You need to be level 80 to enter the instance using the normal entrance. For other means of entering the instance only the normal level requirements apply.", then this would be correct.
If this is not the case, then additional checks in the CanEnter function might be required (or checks have to be moved to CanEnter).
I haven't been able to find clear sources on this matter, some players from blizzard assumed the check would apply for all means of entering the instance.