What people say is not wrong, mate. Chelsea has become an unpredictable club recently. Although they are very bad in the EPL, it doesn't mean that they will also play badly in the UCL. In the first leg at Signal Iduna Park, 1 - 0 is a pretty good result IMO for Chelsea, because they only need 1 - 0 to go to ET at Stamford Bridge.
From the picture of the betting market that you attached, I'm sure that Chelsea will win with a difference of 1 goal, although it's not certain that the match will continue to ET or not.
Chelsea are a great club in Europe since the days they have Abramovic then Mourinho but they did not win Champions League when they had excellent form. They won two Champions League in their club history in seasons when they did not play well in Premier League. Because of their bad performances in Premier League, they sacked their coaches and alternative coaches like Di Matteo, Tuchel brought them to two Champions League.
Di Matteo is even a less famous coach than Potter but yeah I don't know will Potter is capable of making the same thing like Di Matteo after all bad performances of Chelsea so far. Chance to see Chelsea fire Potter before a second leg match with Dortmund is slim but if they fail, they will be out of Champions League. It's a knock out match.
I agree with you, Chelsea always won the Champions League when nobody expected it.
Mourinho did great things with Chelsea, but he couldn't win the Champions League.
And then Chelsea won the Champions League with an unknown, replacement coach, when no one saw them as favorites.
Can history repeat itself? Honestly, the chances of that are very small.
In the Champions League group, Chelsea started very badly, surprisingly they lost the first match in Zagreb against Dinamo and after 2 rounds they were last in the group.
Chelsea then really played very badly, without a clear plan and organization of the game, ineffective in attack, with a bad defense etc.
After that, new coach Potter comes in and raises the level of their game. Chelsea won all the remaining games until the end and won the first place in the group ahead of Milan.
However, Chelsea is again playing very poorly and unconvincingly and I sincerely doubt that they can make any good results this season in the English Championship or the Champions League.