I'm guessing a significant portion of it comes from how terrible the SourceForge experience has been for a very, very long time (I don't know if it's changed, I haven't had to actually use sourceforge in a long time): insufferably slow, brain-dead logical layout of project pages, UI complexity, ...
The kind of feeling which comes from having to use a crappy tool for a long time does not go away easily.
Regardless, SF has been around for a very long time, and one could at least appreciate that they're a pioneer site in open source forge hosting. The humility in the post is pretty evident, and serves as a friendly reminder that this doesn't need to be a competition.
I remember daily offline CVS servers (yeah, I'm that old), unusable slow mailing lists (delays of several days per mail) and the awfully complicated and slow interface. I still can't believe how much slower it even got when SSL was activated.
And the interface is still awful, as is the one of Launchpad BTW.
Still, it was the only open source hosting back then. Many projects were hosted on university servers or even Geocities with no VCS access.
Clearly, without SF Open Source wouldn't be what it is today.
That said, please host your projects on GitHub or Bitbucket in this century. Thank you. ;)
[1] https://twitter.com/#!/ovnicraft/status/65547304698003456