Schizoaffective disorder carries stigma that depression and anxiety don't. People can relate to feeling sad or worried; they cannot relate to experiencing reality differently.
Depression is not "feeling sad" and very much is experiencing reality differently. Flippantly calling something so horrifically painful "feeling sad" is exactly the kind of thing he's accusing everyone else of.
While prevalence IS higher for depression and anxiety, most people have not experienced depression or anxiety and will not in their life.
Lifetime prevalence of depression is 10-20% worldwide, depending on the studies. https://www.nature.com/articles/s41598-018-21243-x
Anxiety prevalence is a little higher but also not experienced by a majority. https://www.tandfonline.com/doi/full/10.31887/DCNS.2015.17.3...
People that have not experienced anxiety and depression can not relate.
Depression and anxiety are not amplified version of emotions. They are not on the same spectrum.
Comparing the disorders makes no sense and only helps stigma.