this is completely false....This is where it gets tricky.
You can have a 2nd or 3rd generation immigrant speak as if they just came off the plane.
It is very very rare for a person to move from one accent to another perfectly
so if a 2nd or 3rd generation immigrant speaks with the accent/language frequently
you will hear it
and that means they have friends, family that speak as such to on a regular
same way if you hang with a bunch of mexicans, africans, caribbean people
you wil EVENTUALLY sound like them or use words they use
if a person says they "born n bred" and have no accent whatsoever
maybe you have to question if they really are
I'm in LA, and personally know countless 2nd gen Americans who have no accent...
very few still have any remnants of an accent...
you're going completely off course with these conclusions