With this first win, at least AC Milan can have better confidence, giving them a chance to top the standings. Lecce is basically not a worthy opponent for AC Milan since they are a mediocre club. The strength of the two teams is certainly not balanced, so I don't think AC Milan strength this season has been fully tested. I think AC Milan true strength can only be seen when they face fellow elite Serie A teams.
However, considering AC Milan is only focused on domestic competition, Allegri job should be a bit easier to get his team back on track. Last season, AC Milan finished eighth, Allegri should be able to guide AC Milan to a top four finish and secure a Champions League spot when the season ends.
You are right that AC Milan win is good for their confidence, but Lecce is not a real test of their strength. The real challenge will come when they face big teams like Inter Milan, Juventus, Napoli, and Atalanta.
Allegri is known for being smart and practical as a coach. Since AC Milan only has to focus on Serie A, it could help them a lot, no European games means less stress and more time to prepare.
After finishing 8th in the table last season, aiming for the top four is realistic if Allegri can keep the team consistent. But Serie A is tough at the top no doubt, Inter Milan and Juventus are strong, Napoli has good quality, and even Roma or Lazio can fight for spots. To succeed, AC Milan must not only beat smaller teams but also show more strength in the bigger teams.
If Allegri builds momentum and avoids too many injuries, Milan has a good chance of finishing in the top four.