[1] https://www.xda-developers.com/xiaomi-anti-rollback-protecti...
Also none of the stock Android phones they sell have a wait time for bootloader unlocking, it's only the ones with their own os MIUI
"The retailers would buy Xiaomi devices in bulk, unlock the bootloader, change the software themselves or flash a custom ROM like Xiaomi.eu (unofficial ROMs based on MIUI China but with more languages and features), and then sell the device. Most consumers would have no way of knowing they’re running unofficial/modified software, and would instead blame Xiaomi for a lack of updates or bugs they encounter."
It's right there on the boot screen. They can make it more explicit if they want with a red skull or whatever. That reason doesn't fly. Also, what changes in 30 days ?
Edit: I suspect Xiaomi do this to keep you using MIUI as long as possible because they make money from showing ads in various parts of the system, and also sync your data back to their cloud which probably is resold in some derived form to advertisers. In my opinion, MIUI sucks. MIUI vs. LineageOS proved it is the phone OS (software) which influences how much a person likes a phone, rather than the hardware.
They've recently increased the wait time from 2 weeks to 2 months which had buyers complaining loudly on forums. So for their newest device (the Pocophone) they reduced the wait time to about 3 days. But I think the long wait times still exist for other phone models.
The wait time depends on Xiaomi's mood-of-the-day. They keep fluctuating it.
And to the Chinese MSS perhaps?