I think it’s just because of the coefficient ranking. Bundesliga will have the Top 5 in next seasons CL because Germany will finish within the Top 2 in coefficient, with Serie A.
So if Dortmund finishes 5th in the league and wins the CL, they will qualify for next seasons CL by winning the competition.
So the 5th spot (for Germany being in the Top 2 for coefficient ranking) will transfer to the 6th spot (if Dortmund finishes 5th and wins the CL)
And no, I don’t believe the same would apply to La Liga (5th getting CL football if Real Madrid wins the competition) because La Liga will be outside the Top 2 for coefficient.
I hope that all made sense.