Lets not misunderstand math.
Swarmlord has Preferred Enemy: Warmaster.
8 out of 9 attacks hit
2 out of 3 of the above wound
3 out of 4 of the above get through Abaddon's save
Total wounds Swarmlord does to Abby per assault phase: 16/9 so roughly 1,78
Abaddon's average attacks
1 out of 2 attacks will hit
35 out of 36 of the above will wound
1 out of 2 of the above gets through save
Total wounds caused roughly 1,78
Abaddon has 1/6 chance per round to do no attacks at all and cause 1/2 wounds to himself, meaning he's doing 1/12 wounds to himself on average per round
Swarmlord has 5 wounds, Abaddon has 4
Swarmlord loses 1,78 wounds per round, Abaddon loses 1,86
Both have I6. Swarmlord does have advantage, even point for point (5 points more expensive than Abaddon).