I took a look at the video (by the way, videos can be embedded so that they show in the post without weighing on the forum) but I couldn't verify that he plays a bass clarinet mouthpiece on tenor (he does have some black thick ligature).
I think that you may have been tricked by the pictures ( and after all what would be the advantage of a bass clarinet mouthpiece on tenor, the chamber size would be absolutely off)
He seems to have tech ( Georg Pfister) make wooden mouthpieces for him but they aren't bass clarinet mouthpieces (large chamber and some sort of wooden ring ligature which may have given you the idea that this was a bass clarinet mouthpiece)
https://www.jazzviews.net/int---trygve-seim---singing-songs-of-rumi.html
A couple of years ago he had started to make wooden tenor mouthpieces because he couldn't find any with the inside shape he was looking for himself. I tried several of his mouthpieces and must admit I had a really hard time in the beginning. But he sent 4 or 5 mouthpieces home with me to check out and after a while I really liked one of them - which I still play. The wood gives a totally different sound to ebonite or metal, and it is closer to my idiom of a sound."
This , I think, is the tech/player with one of his mouthpieces