On Sat, Apr 1, 2017 at 9:22 PM, Niklas Angare via Boost
But if they distribute the whole thing saying "this is licensed under the GPL", doesn't that kind of contradict the requirement to include the Boost Software License and the copyright notices?
It does, if their distributed source (as a package or a VCS snapshot) does not contain BSL with a clear indication of what code is under BSL.
It would feel better to me if they said "this is licensed under the GPL and, in part, other compatible licenses".
It doesn't work like that. A product, as a whole, must be distributed under a single license. (It can be distributed under multiple licenses, e.g. proprietary and copyleft, but that is another off-topic case; each such distribution is independent and whole.) As I understand it, a license, among other things, defines two sets: a set of rights granted to the user, and a set of restrictions or obligations imposed on the user. Two licenses are compatible if the sets defined by the licenses are non-contradictory and can be fulfilled combined. BSL is less restrictive than GPL, and in order for the two of the licenses to be fulfilled the code must be distributed under the GPL. It doesn't make that BSL part on its own licensed under the GPL (or rather, you cannot enforce GPL on that part because the original copyright holders did not allow that).