> This meaning of the phrase, which for Fowler is the original and clearest meaning ... This argument states if an exception exists or has to be stated, then this exception proves that there must be some rule to which the case is an exception

So no, OP used the correct definition and no wide spread usage uses ~Q=>~P. That's a weird logical formulation that was randomly concocted.

https://en.wikipedia.org/wiki/Exception_that_proves_the_rule

Said another way:

Rule: It's hard to find a product that hasn't gotten worse or more expensive, even adjusted for inflation.

Exception: computers i think are more affordable now than even 20 years ago, in bang for buck

This proves the rule because if it's an exception to the rule, therefore the rule must exist in the first place.

That argument only works if someone states the exception from the start, not when someone makes the claim while using the term.