I believe Stallman is credited for this because the average user never heard of Open Source or Free Software until the arrival of the GPL and its enabling of systems built with the Linux kernel and GNU userland.
You make it sound as if it was a coincidence that FLOSS took off at the very precise time that the GPL was created. It was not. The GPL is different from other permissive licenses in that it perpetuates the openness of the system it's used in; therefore it has a natural mechanism for survival of the project, that confers it an competitive advantage and makes it more robust against closed forks.
for whatever reason, FOSS and similar ideas were completely unknown to average users until the GPL took off.
This "whatever" reason may very well be that the GPL was created. The fact that source for all published modifications had to be released back to the project was a strong incentive way back for FLOSS developers to prefer contributing in GPL projects over BSD ones. It may not be that much relevant nowadays because the sharing culture has grown stronger, but the idea of "copyleft" and the "share and share-alike" was a necessary element to make it happen.
The Unix wars were still fresh in our minds, so we were all painfully aware of what happens when Big Megacorp forks a free system and releases a strong closed fork with commercial support, competing with the original project. In that context, copyleft is a guarantee that the software you've volunteered to will not be strangled by a strong vendor using the software without giving anything back.
The idea that big companies should support open source software as proof of their good will, even if the license doesn't require it, was mostly a consequence of the dynamics reinforced by the GPL mindset. Prior to that, BSD licensed UNIX vendors fought over the patents achieved on their particular variant and treated it as intellectual property to be defended, not as a shared resource to be cultivated.