Looks more like a user experience problem than the content creator's problem
Youtube doesn't post process vertical videos correctly, while Facebook, Reddit, Instagram, and many other sites keep the processed video in its flexible aspect ratio - primarily for their mobile users who would have no UX problem.
Interesting how that hasn't made it into the discussion, I feel like people will just stop talking about it when the UX is fixed
Funny you should say that mobile users on Reddit have no UX problem with vertical videos. Here is how the above link looks like on my mobile device. Original link (to classic reddit UI), mobile version horizontal, mobile version portrait, respectively: https://imgur.com/a/JU4v3rn
The video has a full screen button which can be seen in your screenshots. Tap it and the video fills the screen in portrait mode on both the old reddit and the new mobile reddit, at least on my Android device.
I lol'd, I'm reading this on a vertical monitor right now. My secondary monitor is vertical due to cramped desk space, so I can have my horizontal monitor still centered when I look straight ahead.
Developers are a bit different in this regard. I used to use a vertical monitor in 2007 because, well, it's pretty great for big chunks of code and web development in general.
As good as it was, I never wanted to watch videos in that way.