Cantonese is a language, yes, but mutual intelligibility and similarity to other languages is hardly relevant unless the languages are very similar indeed.

For example, there are spoken varieties of English that are mutually unintelligible, while speakers of different Slavic languages are often capable of having a good conversation by speaking slowly and listening carefully.

In practice the main criterion for being a language as opposed to a continuum of dialects is the degree of standardisation. So an example of something that may or may not be a language might be something like Swiss German (but I'm not an expert so I can't guarantee that's a good example). Another type of borderline case is when you have two standardised languages which differ only slightly, for example US English and GB English, or DE German and AT German.

it's about cultural relatedness from a Western centric pov, not language