You are incorrect that some of the mental world has to be the same for every human. This argument clearly shows that we are thinking differently. And yet we can communicate.
If the mental world is similar, it is because it was made similar, because through communication we agreed that something had to have these properties to be a square. But that is an artificial manipulation of the mind, not something that is inherent.
Someone who is partially colorblind, sees colors differently than me. Yet we can agree on what the color red is, even if his impression is very different from mine. So the color red is an abstract concept that has been agreed on, but everybody has their own mapping to their own mental concept of it. It is the same with math.
The notion of an atom is human-tied. The existence of something that we call an atom is not. Your argument might have some value within Solipsism, but that does not make it globally correct or my view incorrect.
Pretty sure by now you either deliberately choose to not follow what is said (eg by me in response to you), or just mix the different consequences of relative (vs total) difference in human-to-human point of view.
That you mention solipsism leads me to conclude that likely both of the above are so, and you can re-read the thread so as to notice why you don't follow the point of it (which is human-particular systems having as an object human-particular notions, next to human-particular systems which also focus on external- ie now even to more degree human-translated from something other- objects.