I think it is important to remember this, because mathematics, like a computer, "fights back." You cannot simply dream up whatever structure you want and have it mean what you want and behave how you want. See Godel's incompleteness theorems. No matter what you are doing, your mathematical constructs (including your implicit Turing Machines in your computer programs) must obey certain underlying constraints that are completely mind-independent. These constraints are what mathematicians study, albeit through a glass, darkly.
Regardless of ontological issues with the post, I like that it emphasizes the designed nature of our mathematical tools. The space of possible tools is so large that there is near-limitless room for human creativity and design in mathematical research. It is a shame that most mathematics classes don't really get that across.
edit: fixed misgendering, sorry, that was sexist.