How to flash a factory image onto a Nexus 5x with locked bootloader?
The only possible way for you to get "OEM unlocked" is to freeze your phone and then turn it on, hoping that it will boot normally again. (Put phone in an airtight plastic bag and place it in freezer for about 1h.)

However, the Nexus 5x bootloop problem is a hardware problem which cannot be fixed using software. (One processor core is broken and as it is turned on during boot, the device crashes.) The XDA work-around simply disables the broken core, I think, but this leaves you with a slower phone.

Since you don't need to save any data on your Nexus, I'd spend my time finding a (cheap? second-hand? lineage-os/treble-ready?) replacement device.