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.
I personally actually hope that Inaki Pena will be able to get the main place in the Barcelona squad after ter stegen injured, while to save their expenses of course there is nothing wrong with promoting a goalkeeper from academy as a cover for Inaki Pena at this time, but indeed the decision to recruit Szczesny is the right action even though it may give rise to internal conflicts between Inaki Pena and Barcelona at this time, because indeed recruiting Szczesny as a replacement for ter stegen shows a great distrust from Barcelona towards the ability he currently have, especially reportedly Szczesny has put forward a condition to become the main goalkeeper if he agrees to join, but regardless of the conflict that may occur later, of course at the moment they need a goalkeeper who does have good skills for La Liga and Champions League competition and Szczesny clearly has everything needed by Barcelona at the moment.