Does that actually provide the bitrate of the video though? And no you just center the image on the screen with "scope" presentations. Note there have been some selections truly in 1:85:1 which will have slight black bars ad the bottom and top but those bars are encoded because they are part of the top and bottom 16 lines (since video is often encoded in 16x16 blocks). There have been some reports that 21:9 features are streamed at 75% of the bitrate of a 16:9 one. It makes total sense to do so.
I think the 10 minute average is probably a pretty good indication of the bit rate. While I agree they could encode widescreen material at lower bit rates - they don't seem to be doing so. Hopefully they are using all the available bandwidth to encode the visible image as well as possible so at least in theory a widescreen title could have better image quality than a 16:9 title.
I wasn't meaning to imply they were wasting bits on encoding the black bars (although it wouldn't take many bits do that compression )