Because you cannot break it with adb scripts. That's exactly why. So if they break it, and they also cannot unlock it (what I assume) - they must be able to load a prepatched firmware onto those devices - which is entirely possible if they have a way to sign it with a key that the bootloader accepts. With all the locks that Vivo has - a simple adb script simply cannot do anything that couldn't be reverted by hard reset.
Now if you find other devices in a closed box that have the same problem, then clearly vivo delivered them broken. But they have not appeared. Only devices sold with open boxes from retailers like TS that tinker with them before delivery so far have been reported to not update. Nothing about it on the Chinese websites. and with 99% of all X90PP staying in China that maks it even more unlikely that if Vivo had a bad batch, that batch would have ended up exclusively at TS... Show me a post where untampered X90PP have this problem and I will entirely say that TS didn't botch the device with tampred firmware. And I never ever claimed they can unlock it (though vivo must have codes/software to allow it - and they could have leaked but I don't assume so)
