Not trying to start an argument but the question was asked in event thread if that would work, the answer was yes. And I can confirm as getting Gjallarhorn as soul reward is how I recieved Heimdallr.
There is no argument. Thor's weapon is Mjolnir, not Thundrous Warhammer. You don't get Mjolnir from the event you get Thundrous Warhammer. Thor is gatcha only.