Another thing is that once you _do_ discover what an icon means, it only takes a few repetitions before you start to remember.
I don't even use Gmail anymore (I've completely switched over to Inbox) but when I opened that article the first thing I did was "read" all the icons on the page. The only ones I had trouble with were "Mark as Read" and "Move to Folder", but even those only took a couple seconds of thought for me to comprehend.
I would probably mostly remember the relative position of the button. So next time when they would add something in-between I would click the third from the left and would be surprised that something else happened.
I have it the same even with labels, truth to be told. I hate that nowadays Google seems to shuffle All, Images, Videos, News around, because I learned that Images are always the second one and then it opens Videos on full moon and News on Wednesday. Now I have to always parse before I tap/click.
"I don't even use Gmail anymore (I've completely switched over to Inbox) but when I opened that article the first thing I did was "read" all the icons on the page."
Well, Inbox shares quite a few of these obscure icons (like the "stop sign with exclamation mark" icon for spam), only they captioned them.
So as an Inbox user you've basically seen the cheat sheet quite a few times. This sort of invalidates your test imho.
I don't even use Gmail anymore (I've completely switched over to Inbox) but when I opened that article the first thing I did was "read" all the icons on the page. The only ones I had trouble with were "Mark as Read" and "Move to Folder", but even those only took a couple seconds of thought for me to comprehend.