The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I'm thinking he saw diving egg-laying adults. I've seen them (Hydropsychids) hitting the water (literally) in the afternoons overlapping with the end of the day's emergence.
My theory as to why many anglers/tyers don't find the patterns very effective is they do not use the correct material (antron, which is hard to find right now) and/or do not tie the fly correctly (touch dubbing method and veil encompassing the entire body).