The /noop command just evaluates the line. The purpose of $readn is to return the last line number that was read from a file. So in order to use the $readn in the script you are wanting he simply evaluated the $read() line to retrieve the number and basically set it so $readn will return that line number later in the script.
If you did do a /noop $readn nothing would happen. You can use /noop to evaluate identifers without having to run a command.