More precisely, some European teams will not take the competition seriously and only consider it as training new players and getting old players used to being able to unite with new players. Munich and Dortmund will definitely use new players at least once in the match and this is a way for the coach to see how the new players perform. But seeing Dortmund from several matches this time does look more serious even though it had previously ended in a draw.
I think it is understandable, perhaps some managers are worried and prefer to avoid the risk of their players getting injured, so they do not take the Club World Cup too seriously. If that happens, it will certainly disrupt their journey towards next season. However, the Club World Cup can be a very good event to build chemistry between players, and also with managers. In addition, each manager can certainly give more playing opportunities to his second string players.
Bayern Munchen have been very impressive so far and are leading Group C, while Dortmund look to be in second place in Group F, but they have the same number of points as Fluminense. Overall, I think both Bundesliga representatives will easily reach the round of 16, both will probably be more serious when they are in the knockout stages.