Excel has one level of nesting within a file, but Excel "programs" are not limited to a single file; and, in fact, Excel has features designed specifically for programs that require multiple files to be open at once to interact.
Its true that each level of nesting in Excel is different in kind so that Excel doesn't support arbitrary nesting of namespaces, and that different bits of Excel functionality support different degrees of nesting.
Excel has one level of nesting within a file, but Excel "programs" are not limited to a single file; and, in fact, Excel has features designed specifically for programs that require multiple files to be open at once to interact.
Its true that each level of nesting in Excel is different in kind so that Excel doesn't support arbitrary nesting of namespaces, and that different bits of Excel functionality support different degrees of nesting.