My co-worker figured out the solution. The directions should read:
Build command: west build -p auto -b qemu_x86_64 samples/drivers/uart/echo_bot
Run command: west build -t run
That is, instead of "-b qemu_x86" use "-b qemu_x86_64". Apparently, the existing documentation works fine with some older zephyr, but at some point the documentation became outdated and stopped working. Zephyr v3.0.0-rc3 needs these updated directions.