And the credit rating one I can certainly get behind but some of it sounds like BS

It is. There's no change to your credit rating or report over whether or not you paid interest. It's not even a reported statistic. At best, you could argue that if they see the credit utilization is low and your payments are always on time that you probably don't pay a lot of interest, but that is a typical indication of good credit.

It's more likely that they had one credit card and not that long of a credit history with it.