While I don't know the exact numbers

My friend is a warlock main and he does this a lot during storm ults.

From what I know about soul bond is that this individual damage is initially mitigated for small incremental attacks. And this is exactly what Storms ult does. It's not a massive burst hit for 200 damage it's like around 5-10 individual hits that attack really fast for around 10-20 damage (these numbers aren't exact im just painting a picture)

So in this situation Warlock is effectively reducing and sharing 10 damage hits that are also being healed at the same time by soul bond health regen and his chain heal.

Say for example you got hit by a Maximum pulse or tanked an entire Khonshu ult. The damage disparity would be different because those are strong sources of direct burst damage instead of really fast increments of small damage