Yes you are right if someone ignores Fluminense maybe it will be very foolish for them. Because Fluminense is playing very well at the moment and they are in very good form. Although Fluminense is a much smaller team compared to Chelsea but with the performance Fluminense is currently in, it will be very difficult for Chelsea to beat Fluminense. We have seen how Fluminense has beaten its opponents and come this far. When playing with Al Hilal and Fluminense I thought that maybe it will not be a very competitive game and Fluminense will lose to Al Hilal very easily. But we all know what happened in the middle of the match.
Fluminense might have a chance to beat Chelsea, we know Chelsea aren't that strong compared to Real Madrid and PSG. But yeah, I don't see anything wrong to bet Fluminense to win in the upcoming match, if they can beat bigger teams, Chelsea could be one of them.
But, if we compare Fluminense with PSG or Real Madrid, they're already on different level.