Many other primates other than hominids throw objects, and as most monkeys would be less intelligent than a Baboon they would surely be Int 2-3. Throwing is as much to do with physiology as intellect, and the Tarrasque is built more like a simian than any of the other Int 3 animals.
Tool use (and throwing something at someone else is tool use) is learned behavior, for primates, for elephants, and even for humans.
The Tarrasque doesn't have other Tarrasques to emulate. And any creature that would use thrown weapons against the Tarrasque wouldn't really be a role model (rather, they would tend to be dead if they were close enough to throw something at a Tarrasque).
If the Tarrasque really is limited to animal intelligence, then it's behavior should be limited. It shouldn't be able to do the things that most untrained animals cannot do and other untrained animals took a long time to learn as a species. It should use its prehensile limbs for digging and other similar behaviors (those behaviors assigned to it by the game designers). A lot of lizards in the animal kingdom have prehensile tails, but they do not use them for throwing. And even when primates and non-human hominids throw things, it's often in a semi-random direction.
Sure, some of the old Godzilla movies had him sometimes acting nearly as smart as a normal human (throwing rocks at other creatures, jumping and hugging itself in joy, etc.), but that's not what should be happening with an animal intelligence 5E monster who is not designed that way. JMO.