So it’s LineageOS.

If you put Michelin tires on a Ferrari, and sell it in that configuration, then it doesn’t turn the tires themself into a sportscar.

…why is MicroG mentioned? It’s not a custom ROM, it’s a GApps replacement for custom roms.

Also, MIUI is not a public custom ROM, it’s the official one for Xiaomi’s devices, also it does feature bloatware, and I don’t think that it’s especially privacy-focussed either.