[OnePlus X General] SebiderSushi:
First off, i'm extremely sorry for my delay! I also happened to notice your message just today.

Right now i got around and tried reproducing your scenario on my own OnePlus X.

As you said that you ran the unmodified setup from the unbrick tool according to my guide, i did as well - and ran into the same issue you were describing.

After some fiddling around, i realized that you must supply the patch0.xml file as well for a complete flash on the OnePlus X when you also modify the GPT (partition table), which the unmodified rawprogram0.xml does. This is not the case if you only install a recovery or other individual partitions so it slipped my mind. I deeply apologize for not testing the command line for the unmodified UnBrick tool package well enough while writing my Guide.

If nothing else is wrong, running

"/path/to/qdl_source_code/qdl prog_emmc_firehose_8974.mbn rawprogram0.xml patch0.xml"

with the unmodified UnBrick tool will fix the device back to a booting state with the stock recovery and Lollipop Bootloader installed on the device., it did so in my case.

Alternatively, if you don't want to reflash all partitions from the package, you can also just try running

"/path/to/qdl_source_code/qdl prog_emmc_firehose_8974.mbn patch0.xml"

Short of any good documentation, i guessed that the problem appeared because the unmodified rawprogram0.xml also writes the GPT table in its last two program elements. If you look in patch0.xml, you can see that it takes care of the GPT in some way. Once i removed the two program items regarding the GPT, rawprogram0.xml could be applied without needing to flash patch0.xml together with it.

So i assume that it is safe to individually flash any partition listed it rawprogram0.xml apart from the GPT. If your GPT is not in a valid state, there's not much booting going on, since your device won't be able to even read your bootloader from the disk without a partition table.

While this implies that you very likely once had an unlocked bootloader to allow installation of TWRP to your device, it is not necessarily the case. For one, it is possible to re-lock the bootloader on the OnePlus X and still boot and use custom recoveries and software. Only flashing images via fastboot becomes impossible again if you relock the bootloader. This is because the OnePlus X is a fairly old device (remember it came out with android 5.1). Such old devices don't support features like Android Verified Boot yet. This is the standard on modern android devices and it implies that a locked bootloader should only load and boot untampered system partitions as signed by the device vendor.

Edit 2022-09-04: I was wrong about this. This only applies to the OxygenOS 2 bootloader. Trying to boot an unsigned ROM or recovery with an unlocked OxygenOS 3 bootloader causes the exact symptoms that were described; The bootloader repeatedly tries booting in an infinite loop. Probably the LOS fash that went wrong caused the bootloader to re-lock, which is why rebooting to recovery didn't work afterwards as well as booting the ROM.

Also, qdl (or any othe software using the Qualcomm Emergency Download Mode) can also install custom Recoveries or ROMs to the devices without unlocking the bootloader and flashing stuff through fastboot.