While its true that native cantonese speakers can generally be identified, its not the case that they lack the ability to speak mandarin. They just speak mandarin with a southern regional accent. Native mandarin speakers from taiwan are also easily distinguished from beijingers.
Generally, cantonese speakers actually pick up mandarin a lot more easily than mandarin speakers pick up cantonese.
Adopting an accent can be a lot harder than adopting a dialect because 1. your brain doesn't parse the new sounds well if you didn't grow up hearing them and 2. you have no experience creating these sounds and its a lot harder to learn to produce them. There's a lot of literature about native accent adoption in second language acquisition that suggests an age dependency due to increased neural plasticity in youth (even though there's a lot of dispute about that critical period for language (not accent) acquisition).
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While its true that native cantonese speakers can generally be identified, its not the case that they lack the ability to speak mandarin. They just speak mandarin with a southern regional accent. Native mandarin speakers from taiwan are also easily distinguished from beijingers.
Generally, cantonese speakers actually pick up mandarin a lot more easily than mandarin speakers pick up cantonese.
Adopting an accent can be a lot harder than adopting a dialect because 1. your brain doesn't parse the new sounds well if you didn't grow up hearing them and 2. you have no experience creating these sounds and its a lot harder to learn to produce them. There's a lot of literature about native accent adoption in second language acquisition that suggests an age dependency due to increased neural plasticity in youth (even though there's a lot of dispute about that critical period for language (not accent) acquisition).