Another way of putting it is that the announcement of WL itself revealed no new information for them, only for us. They could have sat on the WL determinations for a week and did post-II rejections to all those outside of WL group before getting around to the WL announcement. Doing so would have been consistent with their established pattern of pre-WL wave post-II rejections this year.
In the scenario that there were post-II rejections to come, that would necessarily mean there were both post-II rejections before and after the WL announcement. On either side of that announcement, nothing has fundamentally changed from their perspective, only from ours. If post-II rejections and WL are mutually exclusive, why would the former not be complete prior to the announcement of the latter?
It just seems more cumbersome, were it done this way, to have an extraneous "limbo" group of applicants between two mutually exclusive outcomes, when the school had ample opportunity to dispose of them.