This strikes me as fairly unreasonable. The full range of human thought is probably possible to express in the language my sibling and I made up as children, b

This strikes me as fairly unreasonable. The full range of human thought is probably possible to express in the language my sibling and I made up as children, but I'm not arrogant enough to assert that my teachers are all incompetent liars just because they refuse to grade essays written in our made-up language. Every mathematics professor I've ever known would readily agree that part of teaching mathematics is teaching how to communicate ideas using the established linguistic customs of modern mathematics. That's reasonable, just like it's reasonable that literature professors insist on essays written in modern English.