No, it is not. Very nearly all of the code in the Ubercart project was originally written by Ryan or me. The only exceptions I know of are the beginnings of the USPS module, and some small part of uc_cart that Ryan put in.

I don't know why I've started to hear this rumor now, since we've been public for over a year. I hope it stops. I can kind of see why someone would think that if they haven't used one or the other, since they are designed to fulfill the same basic functions, but I would think it obvious from looking at the code that they are not the same.