This discussion is academic at this point, but section 5 doesn't grant any sort of blanket protection to re-using Buildcraft code. Essentially section 5 is saying you can reference the Buildcraft API and build mods that depend on Buildcraft functions/objects yet have a proprietary license. This appears to be designed to address a clause in the GPL, which requires derived (or linked) works like Forestry to be GPL licensed as well, according to many people.
Section 6 is what keeps me from copying Buildcraft's source, renaming it to Makecraft, and distributing it as a closed-source app. Section 6 would also apply if I copied a source file from Buildcraft, altered a few lines, and included it in my mod, which is what I suspected Forestry had done initially.
As far as I can tell, though, the only significant shared code is the API and some third-party code written by Maeyani(?) that's common between the two, so like I said, makes this a moot point.
And I know. tl;dnr/SUMMON WALL OF TEXT/whatever.