It doesn’t matter if he did or didn’t spend any time with Jango anyway. He still has a Kiwi accent approximately 3-4 years later, well after his dad is dead and living alongside reptilian accented Bossk, American accented Aurra, and English accented Dengar. Cut Lawquane kept his Australian(ish) accent after years with his French accented family. Gregor kept his accent after years abandoned on a desert. Bobba’s not trying to hide who he is or anything, if AotC and TCW forms part of the backstory to ESB then Bobba would be very unlikely to have developed an American accent.