I expect this:

@page:left .foo { background: red; }

To apply to instance of class="foo" on left-hand pages only. Should I expect that? I believe I'm seeing that this is not applied at all.

No, this is not applied. There are some properties like "margin-inside" and "margin-outside" that apply to the inside or outside margin of a two page spread, and are therefore determined by whether the current page is left or right facing.