Where in the video do they say "We tested the TV and found it connected to an open wifi network" or anything to that effect?

What they say "Some ISPs provide open networks and some [unspecified] smart devices automatically connect to such networks". They don't say these TVs (or any TVs) are in that bucket. Given how much they've tested about these TVs, I'd say they'd have found and named any TVs that did that.

Honestly this was more a behaviour of very early wifi devices, I don't think I've run into such a device in decades, since people started talking about open wifi networks like they talk about piles of needles.

Honestly there's enough real misbehaviour documented in the video to go without making some up, and it's a little disappointing GN decide to allude to that claim without proof.