There's a difference between a spoken dialects existing and everyone being taught as a kid how to sound letters the same way. There exists a unified way of speaking the language in Spain that if you do everyone understands you.

About the link you linked on inconsistencies with written to spoken.

In general, the orthography of Spanish is such that the pronunciation of most words is unambiguous given their written form

Then it describes inconsistencies that don't exist in Castilian. In Castilian X is always ks, using x as sh is something only done in regional dialects or languages. They even use Mexican as an example when in Spain Spanish, it's written and pronounced Mejicano and Méjico, with the hard J that our grammatical rules would force you to say when reading that word. We now use the x to write Mexico but that's similar to how in English you now write Türkiye, because it's a given name and for respect. Same for Ximenez, I believe that's in Catalan/Valencian and they do pronounce it as sh, but that's another language now is it? In Spanish that surname is written and pronounced Jimenez.

Languages don't care whether there's a "central institution" recording it, because language is constantly negotiated between speakers

Sure, having a common ruleset doesn't help when negotiating /s

The fact that y'all have so many loan words that make your everyday life and you have to learn in a case by case basis of how each word is "supposed" to be pronounced is crazy. In Spanish and other similar Latin based languages we automatically know how to pronounce words we have never heard because the language is unambiguous from written to spoken, no one will look at you weird or tell you you spelled stuff wrong if you follow the ruleset. We even have the ` to indicate the syllable that needs to be stressed so that it's obvious for a reader how to pronounce words.

Yeah the language has changed, sure. But dismissing a central organization that records them and maintains a strong ruleset that incorporates changes is silly.