I think the money aspect might have been the decisive factor here for that transfer. Maybe there was even a deal with the league in the background that Barcelona promised to let certain or several go in exchange for the license for Lewandwoski and the other guys. For Tuchel the transfer is a good one. I expect Aubameyang to play in a central offensive position and that is what Tuchel is currently lacking.
Bottom line is though that I believe Barcelona would have kept Auba if they had the money.
I think have two aspect how reach agreement deal transfer Aubameyang to Chelsea, first aspect Barcelona still difficult and can't registering with new player Jules Koundé after financial problem, but right now after Aubameyang moving to Chelsea they have chance to register Jules Koundé for next match and he has missed two match on La Liga with Barcelona. Second aspect agreement reach by Aubameyang want to play for Chelsea, he realize about less chance become regular player with Barcelona after success get Robert Lewandowski from Bayern Munich. I think good agreement deal between Chelsea and Barcelona, exactly with Chelsea still not find with new central forward after leaving by Lukaku and Timo Warner.