Absolutely.
About 85% of the maids work a shift that covers the time between checkout time and checkin time. After that there is only a limited staff to take care of turndown (if you're in the kind of hotel that does turndown ), bring in extra supplies of towels, bath get and such and do Plat/Gold rooms with guaranteed late checkout.
So it totally makes sense for a hotel to put in a policy by which after a certain time of the day the room gets done, DND or no DND, especially when the sign is up and the room is empty (and believe me, they know it)