You just fell into a logical fallacy. https://en.wikipedia.org/wiki/Denying_the_antecedent

On the other hand, what the other user deducted was:

* If you attend college -> You probably end up writing properly

* OP doesn't write properly -> He probably didn't attend college

Which is a proper logical deduction rule called modus tollens. https://en.wikipedia.org/wiki/Modus_tollens