I guess because to most people, dialect is just a synonym for language which is not. It also has to be because of how they were taught in school.

Another thing, their writing system.

I disagree, writing systems are only a way of writing down spoken language. A language can be written using different writing systems, Malay for example is written using both Latin and Arabic, Serbian is also written using both Cyrillic and Latin but that doesn't make them a different language.

Also, I would like to point out, most Philippine scripts were written on bamboo using a knife so for safety purposes, they hold the bamboo outward and carve their letters bottom-to-top away from their body. After they finished, the bamboo would then be flipped clockwise and read left-to-right just as how we read today.

I don't know about Kulitan but as other historians have mentioned, the Kulitan you see today is not the historical Kulitan used before and was just a recent invention based on East Asian writing styles (hence the direction of writing and the "blocks" of letters for marking syllable coda consonants). Most Indic scripts were never written vertically so it's safe to say that Modern Kulitan is different from historical Kulitan.

Otherwise, I mostly agree with most of your points.