They are not because no unit of measurement is associated with them. 1/2.8" and 1/3" are dimensions because they are associated with " (inches).
I have no idea how to respond to this anymore. I shouldn't even be in this thread because despite the fact I will own the X when it comes out, I don't care what sensor size it has. Someone claimed a type of measurement didn't exist and confused it with f-stop while ironically accusing others of the same thing and I had to respond that it was a valid unit of measurement. I am done. The people who want to claim which camera is actually best should be taking up the comments, not me.
Repeat. WHERE and WHAT are your quoted dimension of 1/2.8" associated with?
Making a mistake of confusing f/2.8 with 1/2.8" is a mistake people make. Coming up with some random dimension that has no association with a iphone 8 that is being discusses is something else.