I think this goes a little deeper as well. Postmarks not only show the date, but the city/town from which the envelope was mailed. It would be possible now to mail something from a small town near the big city where the sorting facility is, and have it look like the envelope was mailed from the city.

Sounds benign? Remember - there are multiple voting precincts in and around a city, and using a city like Austin TX as an example (I lived near there 20+ years ago) - there are multiple Texas counties that surround Austin. Tyler and Williamson counties both in some cases have Austin addresses.

Now someone living in Travis county (a very blue county) could get a ballot for Williamson county (a red county if cheating is fixed) and mail in an absentee ballot for that precinct without any questions on the postmark.

Also (I don't know if this is a real thing or not) if a criminal is confined to their house and they mail something from their own mailbox to the court the envelope could have another city - even another county - postmark on it. Could they now face judicial action for being somewhere they were not allowed to be?

I think this decision will bring many unintended consequences - aside from the intended vote cheating.