This is not at all motivated by surfacing spam.
Wiki and docs posts are bumped based on the theory that edits to those posts are generally of interest.
As for spam protection, the theory now is that these bumps never really provided much extra coverage. Most posts are not the last post, and edits to them were never surfaced by bumps anyway. So if spam via edits is a problem, it needs a different solution no matter what. This was never really an effective solution to that problem.