Touche, Leo K! Perhaps the fates conspired to validate your admiration of Brahms.
Just after I had made the remark about Mahler questioning Brahms, I went back to reading Natalie Bauer-Lechner. Nearly immediately I came upon some remarks that were very complimentary to Brahms, so I guess you were entirely right about how Mahler was really rather vague about Brahms. Probably, it was a case of Mahler's "hot & cold" mood swings. I'd guess he did not like Brahms' attitude toward Wagner, but I doubt that just after hearing Brahms done well, that Mahler would have anything but praise for him--something that's probably true for most of us too!