I would say a secondary type is an attribute to the release group and therefore, in this case, the individual recordings are from soundtracks but the compilation itself is not a soundtrack to a movie. So not a soundtrack (imho).
But what about a single that was taken of a soundtrack ?
or a release that is sort of a soundtrack to a book (booktrack I guess ) ?
or a release that is a compilation of songs inspired by a movie ?
or a transcribed version of a soundtrack work ?
Perhaps it's time to define better what a soundtrack is...