You have to hold your nose in a certain way to get the old Dell to boot the rith efi file, but it seems to work. The new thumb drive will boot in uefi mode and I've been able to install jammy on an ssd with gpt in uefi mode. Update: Spoke too soon on the booting in uefi issue. The problem with trying to switch to uefi with the Optiplex 990 is that I can't get any usb thumb drive to boot in that mode. So there's no problem with the disk creator, but the boot process in the Ubuntu distro is a wee bit intolerant of drive reading speed. I've since verified that using either Rufus (which makes a fat32 partition rather than joliet) or the startup disk creator works fine if I use a faster USB 3 drive. Although its image would verify in etcher and it would actually run (agonizingly slowly) in the 'test disk' qemu session, there must have been a timing problem when booting using the Optiplex 990.
The culprit was the fact that I was using an older USB 2 thumb drive that was reading very slowly. The Universal (Windows/MS DOS) format can be used to install from any Windows or MS DOS environment. It turns out that the problem was not the 'startup disk creator' utility or the iso file. This file format consists of a BIOS executable file.
