Anyone else notice that during this epic battl, Tsun, the greatest warrior to ever walk upon Tamriel, just stands back and watches?

being something more than just a mortal, though not a full deity, he might want to be/forced to be neutral or at least unable to help.

I just think it's hilarious.

He explains why when you meet him :

"A fateful errand. No few have chafed to face the Worm since first he set his soul-snare here at Sovngarde's threshold. But Shor restrained our wrathful onslaught - perhaps, deep counselled, your doom he foresaw."

so yeah, forced to be neutral, likely because of having a spark of divinity.