Except you left out an important bit of the OP's statement, where he/she is talking about Linux's success over BSD specifically.

When Linus was talking about the GPL being part of Linux's success, he was talking about Linux's success in general. And yeah, the GPL most likely is a big part of Linux's success.

However, if 386BSD had been freely available in '91, Linux would likely never have had the opportunity to build the developer base needed to take it from a student's neat project to a viable operating system.