Pretty sure Ult. Mjolnir turned out to be magical, too. Also, it'll probably be said to be less powerful than the 616 version.
He eventually got a magical hammer, but it looked just like regular Mjolnir, it wasn't the same one he originally had. Loeb did it, as I recall. The original hammer, with that shape, was tech based and he eventually got it back when he lost his powers and Stark repowered his old tech suit.
I didn't read the end of the Ultimate line, though, so they could have done another reveal I'm not aware of.
I thought he was always magical? The whole tech thing was Loki fucking around with space time, or whatever he was doing. It was a ruse to get people against thor, at least that was the gist of it in the first ultimates series.
Ultimates 2 did indicate that, you are correct. The Ultimate Thor mini by Hickman filled in his entire backstory and connected the dots.
Thor is a magical being but lost his powers some point before Ultimates series 1. He gets the Braddocks to build him the tech suit and hammer to simulate his powers. After he gets his powers back following the Loki battle in Ultimates 2, he uses the real Mjolnir (which looks like 616 Mjolnir in appearance) and tosses the tech hammer in a locker.
Later, Asgard is attacked and destroyed by The Maker, and we find out that in the Ultimate universe that means the Asgardians lose their powers. Powerless Thor digs his tech stuff out and has Stark fix the tech suit.