It's just a coincidence, nothing more, because Szczesny decided to end his career, and this offer was most likely unexpected for him, but it's hard to refuse. Whether he is better than Ter Stegen or not is a very debatable question, but in this situation the choice is small, so Flick took the path of least resistance, there is a whole season ahead and it was impossible to be left without a goalkeeper. And Lewandowski will help him get used to the team faster.
Szczesny is a goalkeeper, so there is no need to worry about adaptation because his job is only to be a goalkeeper. If the players brought in are defenders, midfielders, or attackers, then they will definitely need to adapt considering that they will be the most important part in implementing the strategy. Szczesny is an experienced goalkeeper, he knows he has to give his best for the club. Although his status is back active after deciding to retire, the offer to play in Barcelona is something he cannot refuse.