Dunno if Tiny Speck even thought of the real meaning of Quoin. I just figure they took the word 'coin' and made up a different spelling for it. Saying Quoin in speech is basically pronounced the same way as Coin.
I asked it and it turns out that Tiny Speck did think of actual quoins (and knows of dentils and balustrades and finials to boot). But, Tiny Speck didn't think that masons or architects or admirers of older buildings would find it too jarring if they were to borrow the spelling.