On a hardware level, yes. Apps don't actually get to access that. Try it yourself in a pitch-black room. Compare black areas in the OEM's always-on display mode to a third-party AOD app or a fullscreen image viewer showing a #000000 box.

I tested on my OnePlus 8 Pro running LineageOS and the first app I found on the Play Store (aodNotify) seems to work as expected. Pure black, as if it was part of the "always on display" feature.

I guess it could work differently on different Android "flavours".

Interesting. I also use aodNotify and on my S20 FE it illuminates the whole screen.

Don't have any OLED Android devices, but on iOS the #000000 box is indistinguishable from off