The reason we went to Github rather than Sourceforge is because of the community. Nethertheless, I think it's fairly foolish to focus on the platform (in the case here of archiving) - it's just a host. Stuff goes on domain 1 instead of domain 2. In either case it's still open source, the archives are all there, git is decentralized and perfect for the job.
Is it just an archive or isn't it? If it is, what's so important about the community, then?
It's fine that you chose what you did, of course. It's just that that in your defending it, you can't seem to decide whether you want to have your cake or eat it.
If you wanted, this would have been a perfectly fine reason to give: "We went with GitHub because it's just what we use mostly, and other stuff not so much."
1. The meat of which remains: if it's just an archive and the host is only being used as dumb storage, how is the "community" aspect of it a plus?