Boy, I can't find this either (but also, the kernel mailing list is _really_ difficult to search). I really remember Linus saying something like "it's not a real SCM, but maybe someone could build one on top of it someday" or something like that, but I cannot figure out how to find that.
You _can_ see, though, that in his first README, he refers to what he's building as not a "real SCM":
Here is what I found based on your lead ("real SCM", from 17 Apr 2005):
> That was really what I always personally saw "git" as, just the plumbing beneath the surface. For example, something like arch, which is based on "patches and tar-balls" (I think darcs is similar in that respect), could use git as a _hell_ of a better "history of tar-balls".
So, I found the git-pasky project in the _very_ early days (like a couple days after Linus's first git commits) and iirc, it was an attempt to build an SCM-like thing over the plumbing that Linus was working on:
You _can_ see, though, that in his first README, he refers to what he's building as not a "real SCM":
https://github.com/git/git/commit/e83c5163316f89bfbde7d9ab23...