However, when confirming that the re-tagging worked:
It took ~1-2 minutes for the list of questions for the "cleaned-out" tag to become empty.
More importantly, even after the list was empty, it still says that there are 2 questions tagged with "clustered-indexing" despite there being none.
I assume it's just some lag due to caching?
UPDATE: Definitely caching, as the count now is down to correct zero