Having to write 40k submissions to find one with 6 8s more than qualifies as rare to my mind.
Automatically incrementing a string might satisfy the requirements, but for an artistic project it's not what I'd base the metrics on, and indeed I don't think the author did.
7c31d274888888f28e37743d4c554e5fffdb7df35a1291bc83119e31743a233b - I adopted 43653 puppies. 5888888c44ce19109d9021679ca8023314cb06a0b48fe9f57f34d2f83ee5a971 - I adopted 321255 puppies. b4b4f1bdf07da6049c7aa172504e8f7464d13333508888889743ebd9136f0345 - I adopted 521564 puppies.
It's hard to buy the classification of rarity outlined in TFA.