I've had this impression I've not been able to shake off for a few years that in the alternative process arena, there's a tendency to use insufficient fixing as a means to harness contrast and/or preserve density that the image or print didn't really have. It's really just an impression and not so much something I can support with solid arguments, except that several times I found alt. process lore to not be entirely accurate in its strict requirements for things like sodium thiosulfate when the ammonium salt proved to work just fine (i.e. in salt prints, Van Dykes etc.) or as in this case, KCN as a wet plate collodion fixer. I can very well imagine that it's possible to retain a little silver when underfixing a plate, and perhaps KCN works to that effect (or fails to work, rather), which will at some point print out and persist as a mild fog, that 'helps' to lift shadows a bit. All I know is that with a pretty bog-standard rapid fixer, I got very clean plates of good contrast.
Coincidentally, I also find it amusing that at the same time, one person alleges that thiosulfate would etch away the silver on a collodion negative (if anything, this would boost contrast by eating away delicate shadow detail), while another (cited) source mentions lower contrast as a result of the use of a thiosulfate fixer, which would imply that either the highlights are reduced while the shadows aren't (which chemical magic would underlie this phenomenon evades me), or that somehow the thiosulfate fixer would quite severely fog the plate, lifting the shadows (which I can very well imagine if insufficient fixing happens, which is a trap that's a lot easier to fall into with sodium thiosulfate as opposed to the quicker ammonium thiosulfate).
Anyway, these are just musing/ramblings; I currently don't shoot wet plate and thus I'm not in a position to do any back to back testing, and I think it's clear to anyone that KCN-fixed plates somehow never caught me in their web of enchantment. Woe is me!
If I ever go back to wet plate, I don't think I'll actually entertain the thought of using a KCN fixer.