Yep, I thought Leverkusen use their Academy players, but when I check their lineup in that match, most of them are senior players
https://www.besoccer.com/match/flamengo-u20-juvenil/bayer-leverkusen/202612247/lineupsI think there's no excuse, even though Leverkusen lost Tah, Frimpong and Wirtz, but the current players should able to beat mediocre team. Unfortunately they lost with big margin, this show if Ten Hag was the problem.
It's just a preseason match, and it's still too early to assess how well the team can play under Ten Hag. I think Ten Hag also needs some time to evaluate the players' form. After all, they had a long break during the off-season, and the team lost several players. But it's only a preseason match, and maybe he just wanted to see what each player is capable of. Of course, I agree that losing even a preseason game with such a score isn't a good sign, but it's still too early to judge their form based on one match.