> If two were used for curly quotes and then one for the straight ones, only one would become available.
Yes, but in fact four slots would have been required for curlies, while two were used for straights. Single and double quotes are two different characters when straight, and two different pairs when curly.
In "straight and curly quotes" it says:
"By replacing the curly opening and closing quotes with ambidextrous straight quotes, two slots became available for other characters."
But surely one slot became available. If two were used for curly quotes and then one for the straight ones, only one would become available.