Maybe it's useful to make a distinction between membership numbers and usage rates? Though it might not explain such a huge change as this one, it's possible that current members are playing a LOT more golf because they aren't traveling, etc., but at the same time clubs are finding that many of the people on their waitlists are uncertain enough financially to opt not to take the membership.
And in this particular case, if Rob Marshall had been waiting for three years, it seems possible that many of the people at the top of the list might have been there for many, many years and now have a completely different life situation, pandemic or not.