the post says:-
Requirements:
1-Having the bootloader unlocked (I tried without unlocking and it did not work for me)
i have stock bootloader so cant use this method
Hmm, thanks for pointing that out. However, flashing stock firmware on a locked bootloader should work if you're updating or reflashing the same firmware onto your device. What a locked bootloader prevents is downgrading the firmware, so I'm not sure what the guide author meant.
As you're updating your device from the soak test, this is the only method to get you updated without unlocking your bootloader. There are no soak test updates captured and you'd have to unlock your bootloader (wiping your device and voiding your warranty) to downgrade back to Nougat. That's unfortunately the problem with soak tests being leaked.
To the best of my knowledge, OPS28.65-36 is the firmware released to Indian devices. Again, you could try to save your data by omitting the fastboot erase userdata command,
but I would strongly recommend backing up your device. There appears to be reports that users upgrading have bugs or stability issues that a factory reset seem to solve. Your experience may vary.