Topic on Talk:Halo 5: Forge
I just recently tested that, so yeah, nice undocumented feature.
However, based on my research (see my OP), unless you got lucky and downloaded the game during its brief Day 0 (day-before-release) availability, the only way to add Halo 5: Forge to your account and, thus, download it separate of the Halo app, is to get it in the bundle. Unless someone can show there's an alternate method, then I say the bundle link is the one which needs to be used since it's the only way to actually get it.