Is there a source for this? Or is it one of those rumours that have been repeated over and over again so many times, that it has come to be accepted as fact?
I doubt the text of the actual licensing agreements between the two companies is available to anyone outside the legal departments of either company.
I also have no doubt that this "rumor" is a true one.
Intel and AMD actually cross-license parts of X86 to each other. Though Intel owns the original X86 ISA in general, people forget that AMD has significant intellectual property and patents of its own related to X86.
Specifically, do recall that AMD essentially created the X86-64 architecture and owns the resulting patents to it. Under the cross licensing agreement, Intel was granted a reciprocal license to use a version of the X86-64 ISA extensions in their own products. Which is why AMD and Intel's processors still have full compatibility with each other running the exact same software despite differing design philosophies.
From the way it was explained to me, that is why the language on license transfer is mutual and not just Intel-dictated (i.e. because both Intel and AMD have IP at risk if an X86 license is ever transferred to another company via acquisition).
Though, to be sure, when Intel signed those agreements I have no doubt Intel had never even conceived of a situation where they (and not AMD) were the company that was at actual risk of being acquired.