The word taʿṣim does not mean that everyone trained in logic becomes incapable of any mistake. A person can misunderstand a premise, neglect a rule or lack evidence about a particular subject.
The commentary highlights the difference between having a means of protection and using it correctly. An error resulting from disregard of a rule does not invalidate the rule itself.
The intended meaning is that logic provides principles capable of guarding thought when observed; it does not guarantee infallibility to every student.