Can’t speak for the parent of your post, so I don’t know if the numbers add up, but population of Indonesia alone is 270 million. And the country is spread out amongst many islands. That’s just one country of many.
There are significant conflicts over water (https://reliefweb.int/report/world/editor-s-pick-10-violent-...). I'd expect those to extend to food too, and be acute in some regions.
I couldn't reply to the other poster (nogabebop23) that was making the point that they don't depend on reefs, but their point is moot because this is a global issue with different impacts at each latitude.