I´d say no, tempest is not affected by the damage reduction rule.
The rule reads:
When a hero performs an attack that targets or affects multiple monsters, the monsters gain an additional advantage during the “Deal Damage” step.[...]
Tempest is not considered to be performing an attack, because you do not follow the usual phases of an attack (declare weapon and target, roll dice, check range, spend surges and deal damage). As such, while tempest potentially deals damage to multiple monsters, it does not do this in a "Deal Damage" step, which only occurs when doing a regular attack. And as such, the rule does not apply.