OK, I got this working. To boot from SSD using the RetroFlag NESPI 4 case, I needed two things. First, I needed to install RetroPie to an 8 GB micro SD card. Then, I needed to run that linux from the SD card to apply the tweaks described in this file here: http://download.retroflag.com/Products/NESPi_4_CASE/How_to_boot_from_SSD.zip Then, I needed to make an image of the SD card on my hard drive, and then write that image to the SSD. Then I could finally put in the SSD and have it boot.
Unfortunately, this means disabling UAS, which greatly reduces the speed of the SSD. It doesn't completely defeat the purpose because it's still faster than the SD card, but not by as much as it should be.This is the first time I got a bad product from RetroFlag. Their GPI case was overpriced but works great. RetroFlag's NESPI 3B+ case was amazing, the best gaming case you could get for the Raspberry PI 3 B+, but I would strongly recommend against purchasing the RetroFlag NESPI 4 case if you plan to use the SSD option at all, because it doesn't work with UAS so you don't get the full speed of the SSD.
I should have just stuck with the Raspberry PI 3B+ and skipped the 4 generation, waiting for the Raspberry PI 5 or 6 or whatever, which should hopefully have proper SSD support and be able to emulate the Gamecube and PS2, cure cancer and solve world hunger someday