I don't work for IBM . . .
. . . but I know quite a few here in the UK and, based on their stories here and news about how their US cousins are treated, it wouldn't surprise me if a large fraction of the workforce were too scared to risk being identified as part of such a thing. As well as large numbers just recognizing that no union is going to make any difference in an organization that treats staff as cannon fodder.
Numbers don't always tell the full story.