Actually it is possible to acquire this imformation on VODs but requires you to make your own browser extension. I prefer using “tampermonkey” which is a browser extension that allows users to write their own scripts with ease via javascript. This extension is available for most modern browsers.
If you have the extension running while the video & chat is being played back, then you can write a script using the browsers “mutation observer api” to watch for any changes made to the chat being played back and extract the username and message.