I've heard this comment a lot over the years and I'm not disputing it though I do sometimes wonder about its origin. Sure, fish eat fish but it's also true that an Arowana will eat anything it can harvest out of the brush overhanging the river and as top feeders they're likely to eat a ton of terrestrial stuff, birds, snakes, lizards, spiders, and according to one account I read years ago, apparently some kind of small primate as well.
Fifty years ago I fed beef heart to fish as if it were an awesome idea and I don't recall any of those fish having significant issues w/ diet, digestion, or disposal. Another example is carnisticks. My fish eat bag after bag of the jumbo and they think it's awesome to the point that they'll often prefer it (by far) over Ron's, Ken's, NLS, etc. but the third, fourth and fifth ingredients are wheat flour, brewer's yeast and corn and it heads on from there to potatoes and rice.
I don't have the answer as to terrestrial byproduct efficacy but I have wondered on occasion if the basis for skepticism might be anecdotal logic.