The solution is to remove the drive and wipe it.
I would file this is a bug except that it's so esoteric and only in support of legacy hardware that it wouldn't make sense.
I will, however, document this for posterity.
Story Time
I was working on an old Dell r720xd and decided to reinstall rather than upgrade.
The plan was to swap out the boot SSD and use ZFS on the new one, so I had to switch the BIOS from Legacy to UEFI mode.
I had already put in the new drive, so I migrated all the instances, backed up some config just in case, and began work over the iDRAC.
That's when ^^^ that happened. I was very confused.
I tried twiddling settings in the BIOS, using a USB rather than Virtual CD and a few other things. It ended up being that once I removed the boot drive, I was able to continue the install.