I think it is much more simple - the power of suggestion.
if I was shown a picture of an explosion and a picture of a flower, then asked "which is more peaceful" I would not be able to pick, because pictures are all equally peaceful. a picture may depict something, but a picture is a picture. what's that? oh you meant to ask about the content of each picture? well now you've completely eliminated any 2D representation of anything, and are now talking solely about explosions and flowers; concepts independent of media. oh but UX person wanted me to think that because an explosion is inherently less peaceful than a flower, that makes a picture of an explosion less peaceful than a picture of a flower? NOPE. totally separate things. try again, UX person.
this is what confuses me. shapes are shapes and have no inherent feeling or opinion about anything. if you want to talk about the concept depicted by each shape then you're no longer talking about the shapes at all; you've left user-experience-ville.
but UX people use a normal person's desire to couple the photograph itself and the subject in the photograph to say "round corners are better, here's a whole article about it" and normal people will (apparently) say things like "sharp corners are so much more jarring for my eyes and make it so much more difficult for light to pass through my pupils so obviously I'm on team round-corner."
just feels like mental slight-of-hand, like someone is trying to trick me into agreeing.
I mean look back at the toolbars on Windows applications from 20 years ago. those icons meant NOTHING (beyond the simple cut, copy and paste icons, really) and we all learned what each meant. and don't you dare try to tell me that a floppy disk icon naturally means "save" and definitely could not also mean "load" or even just "floppy disk" which is what it actually represents. you try depicting the concept of "saving a file to disk" on a button with enough room for a 16x16 image supporting 16 colors.
shapes don't mean ANYTHING. they have no meaning or inherent ease of viewing at all. at best all you can do is hope users either memorize the shape-to-action relationship table or know how to show the tooltips.
I say that UX people are crackpots, but I've worked with a few and they all seemed very sane and well-thought out folks, so I dunno. but then there's this article saying a sunburst shape is "harder to look at" than a circle shape and then it is hard to go anywhere other than going straight to "UX people are nuts!"