This is intended due to its unique passive. Similar to how Black Cleaver is under armor penetration even though it reduces armor, not go through it
It increases magic damage taken, it doesn't reduce magic resist or penetrate it or anything.
Though based on the calculation it could be equivalent to both. If it's magic resist -> abyssal than yeah. But if it's abyssal -> magic resist, nope.
Black Cleaver doesn't penetrate either, but it's in the armor penetration. Also, i never said that Abyssal reduces MR
The assumption I made was:
"It doesn't specifically penetration resistances but it reduce them, that's why it's in that category."
So that's why I talked about the calculation order.