This is completely incorrect.

The rule is that

You cannot generate Steam codes and sell them on alternative storefronts for less than you would sell them for on Steam for a similar length of time.

There is absolutely no rule saying that you cannot sell your game on alternative store fronts for less. There isn't even a rule saying that you can't sell Steam keys on alternative store fronts for less than you are currently selling them of Steam. Just not always selling Steam keys on alternative store fronts without at least sometimes selling them for that price on Steam.