In 5e D&D the fat guy down the local pub who wins every darts competition also wins races, can perform handstands and is able to dodge dragon breath. Your champion rock climber is also a champion weightlifter. And someone who never catches a cold nor feels nauseated after a dodgy takeaway is also a champion marathon runner.
The actual 4e builds that use INT for AC and Reflex defence are wizards, swordmages, psionicists, some invokers, some warlocks, and some warlords. In the fiction, these are characters who use their knowledge and insight to anticipate their foes' attacks. In the mechanics, it's a device for ensuring the right numbers are constructed using the available system inputs - just like a monk's WIS bonus to AC in 3E and 5e, or a barbarian's CON bonus to AC in 5e (I would have thought its STR that gives you muscles of steel; it's not clear to me how having great endurance makes you harder to hit in any literal sense).
By changing it from a class feature to a general rule, all that differs is that there is less need for rules text. The breakdown of the 4e AC system isn't in the use of INT, but in the emergence of classes that don't use heavy armour and don't use INT or DEX as a stat and so need ad hoc, class-specific fixes like the 5e ones for monks and barbarians: STR-based sorcerers, barbarians, etc.