You're delivering the kick with the lower part your fibula. You would also block a low round kick by turning your fibula to meet the fibula of the opponent. The tibia, while potentially stronger than the fibula, is located on the posterior in relation to the Fibula. It would therefore be in no position to deliver a low round kick.
You could deliver a low hook kick, but a hook kick typically makes contact with the heel rather than any part of the leg. If you make contact with the tibia, you are more in a position to deliver a sweep than a true kick.
It isn't As Weak as it looks
Please observe the picture you posted. Notice how the blocker has her foot planted and has all her body structure aligned with the fibula to meet the round kick.
Due to extreme leg conditioning, Muay Thai fighters have dense bones in their legs and their pain receptors are less active.
But it can break
Because of the aforementioned leg conditioning, it is possible to snap the leg of an opponent from delivering a well placed kicked to their fibula. It is also possible to snap your own leg on a well-placed block like the one in the picture you posted. I'm going to go ahead and include this video, but it is hard to watch, so exercise discretion. As far as I know, this is live footage from a MT fight. So, your initial assessment of the weakness of the fibula does have a basis in reality.