Maybe formal spoken Cantonese/Mandarin, but colloquial spoken Cantonese for example is very different from formal written Cantonese. It would sound odd to speak the way things are formally written, if that makes sense.

Perhaps you could say there's a subset of the languages which can be mostly written in a mutually intelligible way. That sounds more like the similarities between, say, Portuguese and Spanish, though, where you can probably write a subset of the languages that is pretty comprehensible by both language speakers, yet the languages are distinct.

that's because people who speak cantonese use mandarin as their written language. theyre not often writing down cantonese