I don't think they are a myth but they are reserved for those that make the most from amway. Those who started the business (e.g. those who own the corp) and those who have large side businesses selling "tools" into the organizations (Yeager, Dornan etc).
I agree that the Amway owners could afford such trappings. It might even be possible for a tenured kingpin to own some big tickets items, possibly paid for in cash. But certainly not the rank and file diamonds that are portrayed as being financially free with all of their toys paid for in cash.
WWDB has at least two (2) diamonds that I know of who lost their homes to foreclosure and one triple diamond who was in bankruptcy proceedings recently.