After discussing with the Support team on a Zoom call today, it looks like the historical attribute tracking only counts users and accounts for who the attribute in question was ever CHANGED in the given time frame. I'm hoping the Product team will invest further into this feature to return the answer to this simple question:
"Give me the MAU for the accounts that were Active=TRUE in the month of September 2022."

Right now it only answers this question:

"Give me the MAU for the accounts that were CHANGED to Active=TRUE sometime in September 2022."

In the screenshot below, the numbers are DRAMATICALLY lower than they should be because the query only returns MAU for Accounts that BECAME Active=TRUE in the last 6-month range.