Some Bluesky asshat politics super-nerd posted it without a source. I googled and I couldn't find it. I did find this though...
[Whistleblower letter to Sen. Richard Blumenthal (D-Conn.)] “When ballots are submitted to USPS in large-volume batches, if any one ballot in the batch cannot be verified against the Portal, all ballots in that batch will be rejected.”
In other words, if there is just one ballot with an issue — like a name that doesn’t match up identically to the name states previously provided to USPS — out of a batch of 10,000, then USPS would refuse to mail all 10,000 of the ballots, not just the flagged one.
AernaLingus's comment has a link to the source.
For clarity - a Bluesky comment
Just to be 100% clear here I just read the source materials and the whistle blower is describing the process for handling ballots that the state is mailing TO voters.
Unless I'm misunderstanding what you were trying to source, it's at the bottom of page 2 of the report that's linked to in that article