It has been mandated by Walter and Andrei that anything official to the d-programming-language org must not rely on any D code outside of it. In particular, whatever program becomes D's official package manager cannot rely on 3rd party libraries. So, anything it needs either needs to be in Phobos or in its own repo. This was brought up with regards to Orbit (another package manager for D), which currently relies on Tango and some other 3rd party stuff.
So, if dub is going to be D's official package manager, it can't be using 3rd party code. Any 3rd party code that it currently relies on or will need to rely on for new functionality (e.g. reading SDL files) needs to either be integrated into dub's repo, or it needs to get through the Phobos review process so that it can be in Phobos. If dub isn't trying to be D's official package manager, then it doesn't matter. 3rd party apps can rely on whatever 3rd party code that they want to rely on, but that doesn't fly for official stuff.
I know the discussion, I also see how depending on Tango is problematic. But it is practically no difference whether code is in the same repository as dub or not, there is no additional library shipped with the core distribution or anything, as dub would link the code in statically.
But it is probably better, to discuss these things at actual inclusion time.