I watched those tournaments and agree we were impressive. We were also, by some distance imo, the strongest side on paper.I think u21's are a reasonable comparison with senior international tournament football. They are players on professional contracts often playing for senior sides, unlike u15's.
I can't find a squad list for that tournament in 2016, but can see goal scorers include Grealish, Ward-Prowse, Redmond, Loftus-Cheek and that Pickford was in goal.
Suggests to me he knows Grealish quite well too, which is a player a lot of contention has been raised about.
Edit - I found the squad lists
2016 Toulon Tournament squads - Wikipedia
en.wikipedia.org
I think Southgate has been blessed in his tenure with England to have the most talented set of English players since 1990.