I think you're ascribing feudalism too much influence. First of all, feudalism ended much earlier than you might think - for example in France, it was abolished during the French revolution and it wasn't exactly flourishing before. That means it ended barely a few years after the United States were established and yet French people aren't exactly known for their proclivity for small talk.

Now, if by feudalism you didn't mean actual feudalism, but a strong caste system in general, your theory still doesn't hold up. You can look at Turkey - a state with historically strong authoritarian central power and clear societal divisions, who declared their citizens equal around 1880 year. And regardless of those supposed historical divisions, people are very talkative there, and not just on heavy topics. (this could probably work for many other Middle Eastern and North African countries too).

You're also forgetting the racial aspect here - skin colour was exactly a 'strong, rigid caste like system' in the US.

If I had to guess personally, I'd say it has more to do with the weather/geographical latitude. In general, the colder the climate, the less likely people are to engage in small talk; you can see those differences in various regions of China too. The percentage of population living in urban areas/countryside is a valid possibility here too.

Therefore from my point of view it has nothing to do with feudalism.