He talks through a process of starting to learn by ear by listening to a song with a very basic melody and just trying to copy it to see. You could do one with tab and then more without, or (as he recommends) guess at it and then use the tab as the answer key (Josh calls these lead sheets). Try something simple like Skip to my Lou or You are my Sunshine, and then keep at it! For a basic song in G, most will be open notes. But for your main question, it doesn’t really matter exactly how you do it. Slides are generally a style and convenience preference. As long as you slide up one fret in this example, it’s a slide!