Hypothetically, could they could be used to 'accidentally" route hundreds of ballots from an important zip code through nowhere Alaska, causing them to be delayed weeks/months and not counted?
Quite possibly, but this would be easily detectable after the fact as standard mail-in voting practices provide end-to-end tracking of the ballot. Most election security measures, including ballot stuffing in most systems, are really only detected after the fact.
I worked for a company that manufactured these machines including electro-mechanical systems as well as the software that used for configuration and generating reports.
The way it works is, these machines would either read the printed barcode or take a picture of the label and run through OCR engine, use the zip+4 and sort them to a bin based on some configuration. Then these bins would go on a truck to their destination.
The machines are not doing the routing. So your fears are unfounded.
How is sorting into a bin that goes onto a truck not routing?
Presumably different bins are set to go onto different trucks, and if the important county elections official's barcode/zip+4 gets binned to alaska, that could cause confusion and delay. More nefarious (and also more unlikely) would be if ballots sent out to zip+4 with registered X party gets binned wrong, delaying receipt of ballots, perhaps.
Although, I would expect there's likely enough volume of returned ballots to fill their own bin, and whoever puts the bins on the truck is going to notice if they're on the wrong truck.