This additional context does help; thanks.

It's still adaptive for animals to avoid all snakes with this coloring (just to be safe)

Yes, this could be adaptive, but not costless. An animal that avoids all snakes that look venomous misses out on some opportunities (e.g., foraging for food in a tree occupied by a harmless but dangerous-seeming snake). The opportunity cost, in reproductive terms, might be negligible, or it might matter, depending on the specifics. (Here I'm agreeing with you when you point to the importance of the term "significantly.")

Because the truth, even in small matters like snake coloration, can make a difference, the original quotation is an overstatement.