I would say they're not feudal since they don't practice serfdom or serve under a living monarch. They are in fact anabaptists who have their origins in the early protestant reformation. What made anabaptists unique when they first came about was their rather subversive (for the time) idea that baptizing infants was meaningless and one was only truly Christian if they consented to baptism once they were old enough to know what baptism meant. If anything, I would say the protestant reformation was part of the larger geopolitical shift in Europe that led from from feudalism to Capitalism, but I wouldn't characterize being Amish as inherently "feudal" or "capitalist." They are deeply religious and communal and live an agrarian and patriarchal lifestyle, which is aesthetically similar to feudalism in many ways, but is not feudal in the economic sense that Marx uses the term when describing the feudal mode of production.