Yep, that's how these work, there was never a SCSI Zip 250 available. I was hoping the adapters would be generic, but the time I tried it, the adapter didn't work with a known good SCSI HDD.
It's not that it's eating the eject commands AFAIK, it's eating the dismount commands at a lower level. I missed your post because I was so excited about finding those IBM SCSI<->IDE units. I'm hoping they will be universal SCSI<->ATAPI adapter at an affordable price point.