Looking at the plugins source it seems you are right.

The video just gets replaced by the plugins text.

I am not sure if this would even save much battery if running in a tab.

No. The extension is not fake. It changes the video source with the audio file URL but it does not blocks the video. The video file is downloaded but it is never used. That's what I said

You can check in the dev tools that when you use audio only extension the video files are downloaded normally. So, it does not save so much bandwidth

But it is a good alternative for Firefox since Music Mode for YouTube is currently only for Chrome

so no alternative one for Firefox? God this is a bad news to me.

There are two extensions in Firefox neither of which blocks the video. Music Mode for YouTube™ is the only extension that blocks the video entirely until now. These two extensions are Youtube Audio (https://addons.mozilla.org/en-US/firefox/addon/youtube-audio...) and Audio Only for YouTube™ (https://addons.mozilla.org/en-US/firefox/addon/audio-only-fo...). The second one is better and the one that recommend. Both have bugs and they do not block the video but they are better from nothing. I plan to create a version of Music Mode for YouTube™ for Firefox but, you know, it is difficult to spend a lot of time for things that you earn nothing.