Tim Henson is one of the collaborators on METAL GALAXY. He and Scott LePage (the other Polyphia guitarist) played the guitars on “Brand New Day”.
It appears to have been a very off-hand experience. They received the request and got sent the track with some dummy guitars and placeholder synth vocals, which he and Scott redid to their style and sent back. They didn’t get to hear it until it came out to the public.
He explained the process on a live talk on Twitch today with Herman Li of Dragonforce (another collaborator of BABYMETAL, he helped with Road of Resistance). Here is the full thing (available for subscribers of Herman Li), and here is a clip of that part:
Just 2 months ago he was hoping BABYMETAL would invite them to play the song live one day:
A month ago he said he thought it deserved a video: