Even though a Perceptual compression sounds as the option to use so it can preserve the visual relationship between colors.
The slider would not be needed in that case?
Or perhaps the DxO devs are quite inventive and cheeky ones and have come up with a very cool volumetric compression of the OOG color space. 
My impression otherwise is that the PSC slider is doing more of a Relative colorimetri
compression of the volume containing the OOG colors. And that the slide adjustment adjust the amount of affected neighbouring colors.
I would love to hear DxO tell us some more.
EDIT:
I did a bit of testing.
Exported the same OOG image in Wide Gamut Space in 4 ways, all with sRGB ICC as Same as soft proofing
Softproof perceptual intent PSC 0
Softproof perceptual intent PSC 100
Softproof relative intent PSC 0
Softproof relative intent PSC 100
I then layered the perceptuals and applied Difference to one layer.
Same with the relatives and applied Difference as well.
Both perceptual and relative compounds did show the same differences as the result
There were NO visual differences to the eye and not according to the histograms for soft proofing with perceptual intent.
I then exported the compounded perceptuals and relatives.
Layered them and applied Difference to them as well.
There were NO visual differences to the eye and not according to the histograms for soft proofing with relative intent
So the PSC slider is fully independent of the intent of the Softproofing - although soft proofing need to be enabled for the PSC to be applied for exported images.
TL;DR
The Intent set in soft proofing do not affect the exported image.
The soft proofing itself to not affect the exported image - which is perfectly correct.
But as the user guide explains - the soft proofing need to be enabled for the PSC to be applied for the exported images.
That might be a factor which adds complexity and might require some refinement or change in the UI.
When reading the explaination in the UI - a user might miss that the PSC is applied on Exported imaged only when Soft proofing is enabled.
Protect saturated colors
For exported images
