I don't think this is something you need to worry about.
Ask at the linuxmint-forum:
and with luck someone will give an easy and reasonable explanation why the deb-name contains that reference.
It is not the first time i have seen that something related to Ubuntu was found in name/infos/other-stuff.
It does not say that the package is not "fully" debian qualified. (one could simply have forgotten to rename it after the build, or perhaps even left that "hint" with purpose).
Also some developers work for Debian and Ubuntu and there sure is -at least kind of- a teamwork in several areas.
There is a debian virtualbox-version, lmde uses the debian repo and you should be able to install it like usual:
Perhaps have a look at /etc/apt/preferences or even /etc/apt/apt.conf
Or install from virtualbox.org, like snowpine said.
Like said: i wouldn't worry about it.