I read that also, but I'm thinking the question at the time of the extension was sign on for $X per point or sign a no thanks letter. Internet conspiracy theories about getting free extensions by refusing to sign weren't prevalent at the time so I don't think many folks were even thinking about that.
And I also recall that DVC offered a small compensation for the "trouble" of returning the documents so it doesn't seem logical that such a large number of folks, many of whom would be too old to care what happens in 2042, would refuse to return the documents. And as DVC members, when someone waves a few dollars under our noses, we do tend to sit up and take notice.
Speculations about topics like this are always interesting, but I usually tend to think the most obvious and likely answer is usually the correct one so I would have serious doubts about the claims that only 50% of the original OKW owners have signed the quit claims.