You should post this in products as it will get more visibility.
It's possible that you are deleting from Marketo but not from your CRM.
Do you mind pasting a screenshot of the campaign you used to delete your records?
Looks like the campaign is still running, but although (overnight) it erased about 19k contacts, the All People and the INACTIVE PEOPLE seem to say they contain exactly the same number as when I began. Should I expect this to go on for a while and the numbers to be updated only after the flow is executed completely?