Good news: I ran the firmware update while having two cables connected to my laptop (USB-C and upstream USB 3) as per the instructions above, the monitor survived and is working fine. Note that after the update, the power button was unresponsive so I could not turn off the monitor before unplugging it, so I went ahead and unplugged it, let it rest for about 30 seconds, plugged it back in and it fired up fine.
Sorry to butt in here but this is good information!
I just purchased one of these from Dell (yet to be shipped), as I have lost some confidence in the old U3011 that's been my workhorse, and I am an avid firmware updater and will use extreme caution with this device as I don't want to blow things up.
I have 2 U3818DW Dell monitors, one purchased from Amazon, the other one from someone on Facebook, without an invoice. I'm aware that the warranty might not be available for the second one, but in case something goes wrong (even a firmware update fails), is there a way to get any help from Dell even if I have to pay for it?
Sorry, but the thought that even a firmware update can break a $1000 monitor is a bit too much.