Fighting other non-boss Pleroma still counts as defending the Citadel, so he wouldn't be punished, no. There is the possibility of damage being done to the Citadel/Achamoth, however, if not enough people contribute to wearing down the boss, but that isn't a consequence resting upon individual character's shoulders and would be a more general worldbuilding/metaplot consequence. It's assumed that some characters need to be focused on the other Pleroma anyway, because otherwise they would be able to run rampant while the other Kenoma are distracted with the boss.

Characters who ICly don't PVP/boss fight at all and just kind of kick back during the raid would get in trouble, but on a OOC level you don't have any threading requirements, as long as Sebastian is said to have been contributing somewhere.