I don't follow. Given that up and down arrows scroll up and down, left and right should scroll left and right, which is what they do.

Left/right - yeah, I see your point. I was testing it on a page that didn't need to scroll left/right when I tried it, so that didn't occur to me. I probably always mouse or trackpad for scrolling anyway, so my key-fu is a bit rusty.

Still... 'delete/backspace' going back in a browser never made sense to me.

On many keyboards, there's an arrow pointing to the left on the backspace key, which may be where the convention comes from.