The shipping routes are in international straights. China would not disrupt them because Chinese ships would be at risk in other waterways they can not project power to.
If you look at the map, China is attempting to claim ownership of said straights.
Your second point is accurate, but China having the ability to disrupt a route brings it up to par with other powers, whereas right now they are solely at the mercy of foreign nations wrt other waterways, with no ability to respond in kind. This gives them that ability.
