But the WGA actually evaluates these things. It's not a matter of Lucas crediting Brackett as a tribute. The WGA looks at the scripts and decides accordingly what the creditation is, in cases of special circumstance like this.
Oh, anyway, it's so long ago, who can say? And do any of us really care? The Empire Strikes Back, for very many of us, is the finest of the six Star Wars films, and I'm glad whoever wrote it wrote it the way they did.