Local idioms and pronunciations exist in all languages, including the Cypriot dialect, and the old man knew them and hence could detect a Kyrenian.
When we moved to Famagusta from Nicosia in 1958 we were shocked at the local pronunciation that turned the Thita sound to H, like "helo" instead of "thelo".
As for the language you learn while young marking you for life, well, actual examples refute it. Greeks were amazed to hear professor Kizilyurek speaking Greek on live TV a few years back. Late president Clerides spoke better "Athenian" Greek than most Athenians, the same is true for EDEK leader Lyssarides. I mention them because their use of the language is recorded in televised interviews.
A few years back we had a reunion of people from our village residing in Greece. Admittedly many of us have been here for decades. The waiters asked us where the Cypriot gathering was, they could not tell from the speech.
A recent study puts forth the idea that we have a data base of sounds in our memory. When we face the challenge of a foreign language we draw on that data to reproduce the sounds. If the exact sound is not in our experience data bank we choose the one nearest, which explains why the Japanese refer to "Itarian restauran" and Greeks are unable to reproduce double consonant sounds in foreign languages. They are a riot speaking Italian and Spanish, languages that often distinguish meaning by pronouncing double consonants. Perro in Spanish is dog, Pero is however, burro in Italian is butter, buro is desk.
As for the standardisation of language deriving from dialects, that is probably true. Greek was "cleaned up" by a movement led by Korais in the 19th century, and a similar movement arose in Italy after unification. The users of the cleaned up language tend to assume a superior attitude to those still using dialects. There is no end to arguments among German, Austrian and Swiss German speakers about the cleanness of the language. Latin American Spanish speakers poke fun at the Castilians for their "lithpi" sounds which were introduced into the language in deference to a king who spoke with a lisp. Portuguese radio was recently accused of racism because it refused to hire Brasilian protuguese speakers. And we all know the contempt British english users feel for American accents.