Not really. You're probably thinking of Project Treble, but there are a couple of caveats to that.

1. Google simply doesn't want the job of maintaining every Android device. They will still rely on manufacturers to update anything that is not Google branded; Treble just makes it easier by separating the base drivers out from the OS, so that manufacturers won't have to reinvent the wheel with every update (and also to minimize the ability of chip makers to fuck over entire classes of device by not providing updated drivers).
2. Treble is only required for devices that ship with Oreo or later. It is optional for devices that are updated to Oreo post-sale, and from what I understand, there are some serious logistical challenges to applying it with a software update (which is why few devices have done so). I don't know if the MiBox (or the Shield, for that matter) supports Treble, but I doubt it would.

It's called project Treble, I've read someone saying they saw the Treble partition so it seems it supports it. Because Oreo doesn't automatically give you Treble, only devices that ship with Oreo do, as reinforced by Google. This reinforcement doesn't exist for upgrading devices.

Treble separates the low level(think drivers) from the OS part making updating way easier faster and cheaper for the manufacturer. However Google doesn't handle updates as far as I know so updates still need to come from Xiaomi, unless some custom rom comes around.

Oreo supports Treble now, after the update. People have confirmed it in other forums.

The Android TV team is also working with its partners to develop a variety of "turnkey" software and hardware configurations so as to promote a more consistent experience for end users, and also to speed up OS updates. Google will begin maintaining generic Android TV images so its OEM partners can streamline development, at least for those who are interested. OEMs who don't want to use Google's ready-built solutions will still be free to go their own way, but the sense I left with is that most of these companies are eager to reduce the overhead of software development and support. Dobjandschi also said that, if all goes to plan, OEMs who opt for these turnkey solutions will be able to distribute much faster and more frequent software updates.

https://www.androidpolice.com/2018/05/08/google-promises-renewed-focus-performance-updates-android-tv/

I'm under the impression that to get full Treble support, all the partitions would've had to have been blown up with the Oreo update? And our data stayed in tact, so...