Are ab exercises necessary for good looking abs, can they be trained exclusively indirectly?
the short answer is: ab-targeting exercises likely aren't necessary, but they're very helpful.
abs are used a lot in heavy leg/back exercises. don't believe me? get your abs nice and sore, then do some heavy-ass good mornings the next day, like 5RM, 8RM heavy. see what you feel.
that said, this is like biceps. biceps are used... probably enough... when doing pull-ups and rows. so are rear delts, for that matter. but when you do pull-ups and rows for years and years, THEN decide to directly target your biceps or rear delts... BOOM! they blow up in size. abs will be similar.
but, like everyone will likely mention, fat on top makes them go invisible. so blah blah, get abs in the kitchen.