Sourceforge became almost useless a long time before GitHub came on the scene though. AFAIR they were taken over by a commercially focused company and then added more advertising and inserted shovelware/malware into downloads.

Again, my recollection (as a non-programmer) was Subversion repos were popular at this time -- launchpad.net linked source code was nearly all SVN -- and the likes of Tucows for MS Windows binaries, and other non-SF sites like Freshmeat became useful again.

Then Linus came out with `git` and the ecosystem shifted.

TL;DR to my recollection there was a gap between SF's decline and GitHub's rise, so the GitHub didn't cause SF's decline?

My recollection is that Sourceforge’s failure to embrace Git was their undoing, and the malware came later as they started circling the drain.

I agree with this recollection, there was a gap period (although that might have been a time when Github was less prominent but still existing)