Android 8.0’s “streaming OS updates” will work even if your phone is full

So they can update even if your phone is full... because it's not actually full, because they already pre-reserved a whole other partition large enough for the entire OS, and all Android 8 does now is patch that other partition directly with the downloaded data instead of staging the patch on the user partition first.

tl;dr: your phone isn't actually full, Google just reserved a bunch of space ahead of time.