Nihility specializes in debuffing the enemy. It appears that different marks applied by characters count as debuffs. Sampo can mark the enemy, making them take wind dot. This can stack up to 5 times. In Genshin, Nahida and Childe can also apply marks to enemies, so perhaps these count as types of debuffs.
The other aspect is the dot from Sampo's mark, turning him into a kind of subdps. But could subdps count as nihility too? Albedo's E creates a 10m radius field. When enemies in the field are hit, the field will do an extra instance of geo damage. This is equivalent to marking all enemies within 10m so that when marked enemies are hit they suffer another instance of geo damage. So it is equivalent to a kind of debuff and could make Albedo a nihility character. Then other subdps units such as Dehya, Nahida, Yae (dot), etc. could also be viewed as nihility characters.
Rather than say subdps is part of nihility or nihility is part of subdps, it may be cleaner to say that the two are different things with some overlap. Nihility is more about debuffing the enemy (res shred, def shred, apply marks). Subdps is more about automatic damage. Where they overlap includes debuffs that deal dot, in which case it is both subdps and nihility.
Besides Sampo, Kafka is another nihility character that can act as a subdps due to her shock debuff dealing dot. The ability to apply debuffs with basic attacks, like what Silver Wolf does, is similar to how hu tao's ca can apply a blood blossom dealing dot every 4s or how Childe can apply riptide with crit hits, making them take an extra instance of hydro damage when hit.
Each of Sampo's attacks can inflict wind shear, which deals dot for 3 turns, stacking up to 5 times per enemy. Sampo's skill is a bounce (richochet) that deals damage to one selected enemy and then 4 random enemies. His ultimate deals aoe damage and increases the scaling of the dot on enemies for 2 turns. With a passive trace, enemies affected by this debuff deal 15% less damage to Sampo.
In Genshin, Sampo's crit hits can apply a mark to an enemy, stacking up to 5 times. (Genshin does not have effect hit rate, but can use crit rate like Childe's riptide.) The debuff lasts say 8-15s. The more stacks of this debuff the enemy has, the more damage over time it takes. The debuff deals wind damage say every 1-2s.
Sampo's E is a bounce skill (like Beidou's Q or baizhu's E) hitting up to 4 enemies. His Q does an aoe anemo nuke and increases the damage dealt by the debuff by say 40% for 10s.
In Genshin, Sampo could be a subdps character that deals off-field damage.