YUROP

It's a win for the party, but a win for the party is different from winning the election.

In the last Australian federal election, the Greens quadrupled their number of seats. It was absolutely a huge win for the Greens. But going from 0.7% of seats to 2.6% cannot mean you "won" the election. (Also...wow...that shows just how gross single winner elections are. Even with preferential voting. When a party that consistently gets over 10% of the votes is able to win less than 3% of seats and call that a huge win. Proportional systems like Germany's MMP are amazing!)

Whether you want to say the CDU/CSU "won" the German election, IMO, depends less on how their vote changed relative to the last election, and more on whether you want to say the party that ends up selecting the Chancellor "won" an election, even if they need to go into a three party coalition. My personal take is that yes, it's not unreasonable to say they won.