For future reference, I actually took some time to look at what Krita might be doing differently from scribus… and the answer is that it isn’t doing anything different from scribus and whoever told me that was probably confused:
(Both images softproofed with the same profile, with the same gamut warnings. As you might be able to tell from the bright green gamut warning on the black, the blacks are also faded in scribus)
EDIT: To clarify: The softproof is two transforms (image → softproof → monitor) while the regular one is only one transform (image → monitor). For the second transform, softproofing doesn’t turn on blackpoint compensation. So the result is identical if you turn off black point compensation in the settings (and fiddle with the monitor profile briefly, because apparantly the display transform doesn’t get changed when bp compensation is toggled, this one is a bug).
One of these is converted to CMYK chemical proof, the other only soft-proofed. Blackpoint compensation has been turned off in the display color management settings.