Skirmishes do not count for any mission unless they are specifically named.
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that is what it says in the skirmish help link in the plunder screen but earlier i skirmished a player listed in the list and got no points

is there a bug here or did i not understand it right

I had the same problem, but no bug, it does mean that unless it states in the mission that you can skirmish your targets, skirmish attacks will not count towards your mission.