You seem to be conflating whether or not someone knows about a tragedy with whether or not decorative circles are immediately seen by everyone, literally everyone, as a sign of the devil.
Let's say you lived in the middle ages and eventually heard news that some great faraway land named China had blown up. Well, you'd probably hear a lot of things about it, some things with elements of the truth, other things stretched by fiction, fear, myth, and tall tales.
But that doesn't mean you suddenly know the secret handshake of a secret society.
And we're back to "there should be a Religion check to recognize SH's symbols, but since Tav should at least know about Shar (stop thinking in small-terms, the Spellplague is more like if Shar took out electricity for years in our real world), we the player should be informed prior to SH's reveal."
Or even more at least, if we the player aren't told about Shar, then Tav's dialogue options should include an option: "Who's Shar? I lived under a rock for my whole life."
