But weren't most recordings manually started in the video? Either through the UI, or via the root access they have?

I might be missing something here, but I did not see the TV automatically recording stuff by itself (or them showing it is transmitted somewhere) without a visible UI element showing that the microphone is active.

It seems that they turned the TV into a wiretap via root. That is a concern and might give some lessons for LG that they need to invest more into security of their devices (privacy indicators, attested boot, anti-root / anti-persistence measures, hard-shutting off the microphone without privacy indicators)...

But you can pull of similar attacks with a compromised laptop / computer or smartphone.

Of course, it's a TV, why does it need that stuff? Still though, I think the video itself is a bit misleading that it claims (or many people think) it automatically records and transmits all this stuff, while this was not shown here (unless I miss something).

My takeaway was just that the root access gave them the ability to find out what was being recorded, and they were able to play back recordings that it made.

It didn't appear to me that they were initiating anything.