I'm confused. I've always used justify- in grid as a single axis thing.
In that MDN page, it says "In grid layouts, it aligns an item inside its grid area on the inline axis."
And on the grid alignment page: "To align things on the inline axis you use the properties that start with justify-, justify-content, justify-items and justify-self."
Nothing about both axes at once.
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_box_alignment/Box_alignment_in_grid_layout