That's just a fan's hope, but good results for Leverkusen are possible in his first season as long as Leverkusen has the best players capable of outperforming Bayern Munich's players. Because Munich has truly dominated the Bundesliga, there's a huge difference compared to the Premier League. When Ten Hag was managing Manchester United, he wasn't supported by better players, especially after Ronaldo's move to the Saudi Arabian League, which led to Manchester United's even worse season.
To beat Bayern, Leverkusen would need players who are just as good or even better because Bayern long term success comes from having and keeping top players which creates big difference compared to other Bundesliga teams. This is different from Premier League where many rich clubs share best players making competition for title closer. Your example of Erik ten Hag at Manchester United shows that even good manager will struggle if they do not have enough talented players to compete at highest level. When key players leave or are not replaced it can really hurt team performance no matter how good manager is. At end while fans hope for best team lasting success especially against dominant opponent really depends on skill of its players.