This statement seems to be wrong, as there are at least three languages that are not dialects of Castellano here (likely more)
Afaik, castellano is spoken everywhere, at least as a secondary language. Hence I understand the map to show regional variants of castellano spoken either exclusively or secundary to another local language.
Edit: precised my input a bit since it led to misunderstandandings
If 100 % of Indians start using English as their lingua franca (they’re on a track to just that), does that make Hindi a dialect of English?
The sociopolitical reality of a lingua franca does not define the scientific linguistic reality of other languages.
I will say that personally the notion of catalan being a subset of castellan sounds ridiculous on account of the fact that in its written form catalan is roughly mutually intelligible with french, where castellan is not. If it’s going to be lumped in as a dialect of something, it’d be more intellectually honest to make it a dialect of French.
