The victory of Leverkusen as Bundlesliga champions last season was magical which has boosted their confidence in the league but about this new season it's very obvious with how they are starting off that they can't repeat the spell of retaining the title no matter how they try because other opponents like Bayern Munich, Brussia Dortmund and RB Leipzig look very set and prepared for them, especially Bayern Munich because to them it's somehow a challenge that Leverkusen had to strip them of their birth right and letting Leverkusen retaining it this season is very unlikely.
Bayer Leverkusen still has the oppurtunitty win the Bundesliga because the league is still young. They still have the quality players since they retained most players who made them champions last season. What is certain now is that they will not maintain their unbeaten run this season since they have already lost a game. The Xabi Alonso side was defeated by RB Leipzig making them to lose three points. The league is competitive with good sides like Bayern Munich with some of the best players in Europe. The surprising leader of the table now is Heidenheim, while Leverkusen is in the eighth position.
The opportunity is not just open to Bayer Leverkusen only but to every other teams in the league which is what is raising the dust about Leverkusen going through an early defeat for a team that had an unbeaten streak all throughout the last season. Fair enough to say that they may not be able to repeat an unbeaten streak as it were but basically the early defeat just at the start of the season can affect their winning mentality.
However, I do agree with you that the season is still fresh and there's a much window of opportunity for the Xabi Alonso side to restrategize, not putting their guard down in order that they may prevail because it's really a challenging season as other teams ain't smiling which we will be seeing many surprising results this season in the Bundesliga.