I don't know where this idea comes from, because if he is playing at a lower level, then the benefits are not there.
As it is, he's (apparently) training with the senior squad, which, given the players we have, would seem to me to be much better for his development.
The idea comes from the likes of, Sporting Lisbon, Porto, Ajax, PSV, Monaco, Southampton, Anderlecht, Lyon, Brazilian clubs etc etc where lesser clubs have given ample opportunity for kids to develop in first team football and then sold later on to bigger clubs where they have become exceptional/world class players.
Plus, too many promising kids have been fu**ed up by playing first team football too early in weak leagues.
First team football is often overrated in a young players development.
And one could make the argument for promising kids being fu**ed by not playing at an earlier age with first team football more beneficial than training with first team players.
Personally, If I were 16-18, I would choose 15-25 games a season at a club like, Southampton, Ajax, Dortmund , Lyon, Monaco etc over 3-4 games and training with seniors at a big club.