There are several ways to make an HDRI probe image.  The key point of HDRI is in its acronym: High Dynamic Range Image.  The key isn't in the 360d environment, but in that the image stores more than just RGB intensities.  It also stores luminance intensities - from which it is possible to gain realistic lighting models applied to the 3D scene.
Most HDRI is applied fully spherically.  Truespace seems to be making unqualified assumptions if that is how it does this - maybe there are options (?).

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

Contact Me | Kuroyume's DevelopmentZone