Answer:
The difference of dialect is not the issue. The Latin connection is.
Explanation:
To start with, we can assume that the American doesn´t speak a Spanish dialect, because dialects are generally spoken by minorities in their own home country. The reason why the truck driver believes the American speaks an Italian dialect is simple: it´s due to the fact that Spanish is a Latin lenguage, i.e. the lenguage spoken by the Romans (first only in Rome, then all over Italy during the beginning of the Roman Republic and by 117 AD the Roman Empire and it´s lenguage was all over Europe). The similarity of Spanish and Italian is a bit like Dutch and German, because Dutch has German roots, whereas Spanish has Latin roots.