Yes, I am stupid and it happened to me.
Now, as I read here on XDA, there are some people who managed to brick their device by triggering the ARB when they converted the Mi A2 to Mi 6X to test MIUI. So if you flashed MIUI 10 and didn't deleted firmware-update from the .zip file, you triggered the ARB. If you tried to flash MIUI 9 or stock Mi A2 ROM, you managed to brick your device and it stays in an EDL state (Qualcomm 9008). You SIMPLY CANNOT rollback to Android One yet but you can revive your device very easy without test point or Mi Authorized Account.
Steps are simple. You just need to restore the device to the latest available MIUI 10 with a modified version of MiFlash by using wayne (Mi 6X) ROM with patched files in order to bypass the Unauthorized Account thing.
Keep in mind that your device will be MI 6X forever! Forget about Android One! If you want to revert back to stock Android, flash AEX or Pixel Experience.
1. Download MiFlashTool and Patched Programmer files: CLICK
2. Download latest Mi 6X MIUI Developer ROM: CLICK
3. Unzip all the files on your Desktop
4. Copy the Patched Programmer files to images folder of ROM
5. Start MiFlashTool, select the folder with your ROM (do NOT select images folder), on the bottom-right select flash_all
6. Plug the phone on your computer, hit refresh and flash fast!
7. Your phone will start flashing. When "success" appears, unplug your device, press the power button for about 15 seconds and boom, it's alive again!
8. If you want root or to flash another ROM, download the TWRP image from the folder above and type:
fastboot boot «drop here the TWRP image»
Once booted, copy the TWRP image to internal memory
In TWRP, go to Install -> Install Image -> Select TWRP image -> Recovery
Reboot -> Recovery
Since this device is no longer a Mi A2, we have now a standalone recovery partition. With ARB triggered, you can flash any ROM from TWRP (MIUI 10, Pixel Experience, AEX, Treble etc.). If you flash again MIUI 9, you will brick again your device.
If you found this tutorial helpful, hit that thanks button!