hebele I see. This is the reason I didn't find the slide in his video, because I was curious about Christians finger positions. (Not that I would change mine, if his would be different. Nope - I even change his fingering numbers too often.)
But it is slideable. It was instantly in my muscle memory to be recalled. Therefore, I guess he did the royal thirds with that slide somewhat later. Probably around lesson 10 or later. In lesson 2 definitely I didn't play this slide, because I only followed the video.
Maybe you should skip this specific slide for now and go on. If it was the comfy slide without the need for the curled fingers, I would encourage you to do it. Don't worry - you will learn it later, and you are missing nothing for now.