If we can believe the tech support person quoted here, DxO uses Adobe RGB (1998) as a processing color space. It has of course a fixed gamut. Colors that can be created and fall outside Adobe RGB (1998) gamut will get clipped.
An example of how ProPhoto RGB can
be used within a raw processing pipeline, and the gamut implications can be seen here with REAL images:Everything you thought you wanted to know about color gamut
A pretty exhaustive 37 minute video examining the color gamut of RGB working spaces, images and output color spaces. All plotted in 2D and 3D to illustrate color gamut.
High resolution:http: //digitaldog.net/files/ColorGamut.mov
Low Res (YouTube): http://www.youtube.com/watch?v=n0bxSD-Xx-Q
This video in no way speaks to what DxO is or isn't doing since we have no idea of the technical correctness of the tech support post concerning color management in general. Certainly their take on ProPhoto RGB and bit depth seems quite nonsensical. But point of the video, (which was generated to squash some miss understandings about using Adobe RGB or worse, sRGB instead of raw and using a larger processing and encoding space) is when the goal is to gather and possibly use as much possible colors a digital camera might produce, nether is adequate in terms of gamut size. Colors get clipped.
As pointed out below, ProPhoto RGB is simply one possible container
for our image data. In no way do we need to think we can or must fill it with data. The Adobe RGB (1998) container is too small if the goal is to encode all the color gamut potential
of our images.
DxO isn't the only converter to use Adobe RGB (1998) not that this is ideal. I'm pretty sure that Apple's Aperture does as well.