I happened to watch a segment where the cars on the opposite lanes looked more like passing on an independent road, so I missed that this was meant to be lefthand traffic. And it really looked more like an interstate passing through a city. – My failure.

He means cars overtaking you using the lane to your right. In Europe this is against the rules.

Yes, but the cars are driving on the left in this demo (like in the UK), so on the right is where you do overtake legally - in the faster lanes. I haven't noticed any "overtaking" on the left, where it would be illegal in some countries that drive on the left in the few minutes I've watched it.