I don't know what the designers had in mind, maybe they intended it to work one-handed and maybe they didn't. It took me a little while after buying one to figure it out, but I always close mine one-handed now and in fact I think it is one of my easiest knives to close one-handed.
I just tried closing mine and see that I normally put my little finger lightly against the clip. I tried intentionally not touching the clip with my fingers and find that I can still close it easily one-handed.