OK. I haven’t gone through this skills course but I looked at it to see if I could help. I think the confusion is because the music is in another tuning. I didn’t notice that at first until Tony kept saying to rest the thumb on the G string which he placed on the A string.
Normal tuning : strings= 6th-E, 5th-A, 4th-D, 3rd-G, 2nd-B, 1st- E.
Drop G tuning: Stings=. 6th-D, 5th- G, 4th-D, 3rd-G, 2nd- B, 1st- D
What I would do is look at the tabs. The tabs will show you what fingers and thumb to to play on what string. The index finger plays the 2nd fret of the B string (2nd string) and then hammer on the 3rd fret of B string (2nd string). Then the middle finger strikes the 1st string which in normal tuning is the high E string. Because you are in open G tuning the 1st string is tuned down to a D which is where the confusion comes in. Tony is calling the 1st string a D because it’s has now become a D per the open G tuning. Hope that helps. Kim