Well, we know what this means. 42-40 means that they don't push leaners at all, if 18% of the public doesn't answer. What
that means is that the poll is showing 42% more or less solid support of SSM, 40% opposition, and 18% wavering. This is totally believable to me, working under the assumption that, when push comes to shove the 18% will go "meh" and walk away rather than passionately oppose it. Post-Obgerfell, apathy is de facto support of SSM, much like pre-Obgerfell, apathy was de facto opposition to SSM: status quo is the result of apathy and national SSM is the status quo now.
40% of the population strongly opposing SSM makes sense alongside earlier polls.