No it's not a closed shop, quite true. I have been approached this week by a few of the newer staff worried about going to work next Tuesday in case nobody talks to them, I just tell them turn up if you wish. And to be fair, everyone has told them the same.
You should’ve told them that the majority of their colleagues won’t support them in future if they find themselves in need of union assistance and let them know the reality of deciding to work through a strike.
They‘d be as well saving themselves the union fees in the meantime.