If that's so (not saying it isn't), then Linux Mint should simply offer an AIO download that will allow the version one wants.
Kind of like my Windows 7 64 bit AIO DVD, provided the proper license key for the selected OS matches, can install any version, there's another that has all of the 32 & 64 bit versions in one ISO. This is a legit workaround for when retail ISO's or DVD's won't activate an OEM, Upgrade or Refurbished PC COA, the AIO handles all, just by one simple mod.
All Linux distros should be releasing these, if more than one version is offered, to select version at time of install. If OSDisc is doing this, then they've modified the ISO to give the user the choice of version to install, though can't be including Ubuntu as dannyboy stated, nor Debian, like Ubuntu, it's entirely different. Will need to research this Mint AIO install media, there would have to be a published workaround (or one available for download).
OSDisc is only sharing a third of the profits with Linux Mint, which can reduce any donation to Mint big time, to probably less than $1 per DVD. While I donate to Linux Mint, only do so by the provided link, not by purchasing things where only a fraction of the proceeds goes to the distro.
It would seem that Linux Mint would offer a 4 OS combined ISO (Cinnamon, KDE, MATE & Xfce) if that's the way they wanted it to be distributed. By a 3rd party doing this on their behalf, may be altering key important files, which may affect performance, operation & other aspects of the OS. OSDisc.com is a 'jack of all trades' operation, with no promise that all of their Linux based tools are the latest available. May as well purchase on eBay.
At any rate, it would be a total waste of cash to purchase now, with Ubuntu & all of it's derivatives soon getting an upgrade, it'll be another 2-3 months before Mint 18 ships, and longer with KDE/Xfce, as both of these are not under the control of Ubuntu nor Canonical.