didn’t you have to backup a partition to be able to relock the BL on the X?
I think you’re remembering the Trim Area (TA partition), this partition will self destruct as part of the official unlocking procedure.
This partition holds a couple of cryptographic keys which are used as part of the DRM scheme and (on older Xperia X firmware versions) also to unlock some of the advanced custom image processing that Sony has developed for the camera.
Restoring this partition is not necessary to relock the boot-loader (you can relock by just reflashing a complete brand new clean Sony official firmware including all the partition), but not restoring it means that you will miss the crypto key and will have reduced DRM capabilities (a different widevidecdn level on netflix ?)
(And this was doable due to a flaw in the earlier version of Android which is not possible on the later devices.)
Indeed if you want to back it up yourself, you need to gain root access on android before unlocking the boot loader (and thus before TA self-destructs). On Xperia X this was done by using the “Dirty CoW” exploit, I’ve done it just in case, but at no point did the repair subcontractor ask for it.
For my next Xperia XA Plus upgrade, I didn’t back this up, as at that time there wasn’t any useful exploit and given that the repairs never asked for it, I didn’t see the point. (by the time I finish using that one, it will be considered “old hardware” and nobody will bother buying it second hand anyway, so nobody will miss the DRM keys).
I presume they have other software cable of relocking the BL?
In theory, as an official repair subcontractor, they could have had access to official Sony tools and be able to reflash the TA partition with a new official key.
Or maybe they just relocked the thing as-is.
Frankly, I avoid DRM like plague so I haven’t gotten to occasion to check if my widevinecdn level has been degraded.
If the contractor doesn’t have the tools the last possibility to restore DRM without a backup is to swap the motherboard with a new one from Sony (who in turn have the flash tools to refurbish it), as @toanja has reported.
But in my case (replacing a separate daughter board that handles charging), it doesn’t make sense the DRM keys are necessary.