That study makes no mention of G. oregonensis, G. unicolor, and G. venenata.

show that Galerina “marginata” is a complex of (probably) different species:
“http://www.mycologia.org/...;

Mating tests between some of them have also shown incompatibility, but marginata/autumnalis were compatible according to this study.

“What is now recognized as a single morphologically variable taxon named Galerina marginata was once split into five distinct species. Norwegian mycologist Gro Gulden and colleagues concluded that all five represented the same species after comparing the DNA sequences of the internal transcribed spacer region of ribosomal DNA for various North American and European specimens in Galerina section Naucoriopsis. The results showed no genetic differences between G. marginata and G. autumnalis, G. oregonensis, G. unicolor, and G. venenata, thus reducing all these names to synonymy.”

Gulden G, Dunham S, Stockman J. (2001). “DNA studies in the Galerina marginata complex”. Mycological Research 105 (4): 432–40.

http://www.sciencedirect.com/...

I don’t see any clear explanation why this was synonymized with Galerina marginata/G. autumnalis. Neither Index/Species Fungorum or MycoBank support synonymy at the moment (Dec. 2011).