The instant in time at which your letter is technically "received" by the USPS, in the sense of them having physical possession, has never mattered for any legal purposes whatsoever. That's because in many cases, there is absolutely no physical proof of exactly what instant in time that event happened.
Electoral rules aren't written based on when the USPS received your ballot, because that's typically unprovable. They're either based on when the ballot was delivered to the election office, or when the ballot was postmarked. The postmarking may happen at some point while the piece of mail is in USPS custody, or it may not happen at all if you don't specifically ask for it.
The rules are based on the postmark date because the postmark date is the only available documentary evidence of the date of mailing.
Again, this is not a change in policy, it's merely documenting the way the mail system has always worked.