Your logic is incomplete. You failed to take into account or do not understand all the behavioral aspects of the snakes that employ the discussed defense strategy.Startle displays are, now I hope you can follow my logic because it's apparently difficult for you, are meant to startle. Birds can be highly neophobic, and thus a startle display can give them enough pause for a snake to escape. This is likely a target for birds as they are sensitive to long wavelengths (reds, oranges, yellows), which you'll note, is what the ventral colors of ringnecks are.
Snakes that use the tail flashing defense use it any situation where they feel threatened. They do not always simply flash their bright colors to buy time and then try to run. They display these colors often times with tail wagging or little movement and patiently maintain the pose until the perceived threat is determined to be gone. The predator is not distracted or startled while the snake escapes. The snake holds it ground. The predator questions the safety of latching on to its potential meal and abandons it. Its a behavior that would be more useful against a predator that stalks its prey as opposed to visual quick strike from above predator in which case the snakes cryptic dark top coloration would be of more value. This behavior also cast serious doubt on the "Disruptive coloration. It's possible, for sure" theory as it was categorized.
"Grizzly Bears eat fish, but I'm not about to use them as a model of coral reef fish predation. "
That is smart of you because Grizzly Bears don't inhabit coral reefs. But again the analogy falls flat because the members of the mammalian taxon I listed are voracious snake predators to be sure, coexist in the same habitats and have life styles that would allow them to have frequent interaction's with "Tri color snakes". The use of these mammal's as a model for mammalian tri color snake predation encounters makes perfect sense. Grizzly Bears eating reef fish not so much.