I don't think the point is to memorize the song (though if you practice it enough times, you will inevitably have it memorized). It's moreso to get you used to reading tablature and learning how to put your fingers where they need to be. That's the case for me at least. I'm quite far along in the fundamentals course (chord power pack section), and I can't really remember how each song of the previous lessons went, but I definitely got what I needed out of those lessons. I know how to read tablature, how to play basic tabs, simple chords, regular chords, etc without looking at the screen or guitar. There are a few songs that I memorized because I played them quite a bit so you don't have to worry about forgetting them lol. It does help to train yourself to not look at the tabs or even your guitar while playing so you can build your muscle memory. But it's best to initially go at a slow pace while looking at the music. Then once you're able to play fluidly at that slow pace, start to test yourself at a faster pace or without reading the tabs. It all takes a lot of time and repetition to get to that point of course.

tl;dr - i'd say it's safe to move on if you feel comfortable doing so like john said. the tabs don't have to be memorized. the point of the lesson is to help you understand how to read tabs and how to play melodies. if you ever need a refresher on something, you can always go back.