In general, halahca isn’t decided by oral reports of what Gedolim said (see Igros Chazon Ish), especially if didn’t explicitly state that they were expressing their opinion to be accepted “halacha limaaseh.”
Additionally, the general rule is that we follow Rishonim over Achronim. Even those who would follow the Achronim over the Rishonim, that is only if the Achronim are aware of the Rishonim. So if the Brisker Rav wrote: “I am aware that the Rambam held that hishtadlus affects one’s shidduch, but I hold halacha limaaseh that we should not follow the Rambam” it would be one thing. Being that he did not quote the Rambam’s opinion, we have to assume that if the Brisker Rav was aware of the Rambam’s opinion he would have changed his mind regarding this issue.