Hi, thanks for translating the SyntaxWML page.
I'm not sure, but I think you may have misunderstood this sentence: "An explicit index inside an array is also considered a container."
Allow me to elaborate.
The attribute variable="foo" references an explicit index (index=1) inside an array. When expressed in this form, it is always a Container variable. This means, for example, that it would not make sense for an arbitrary array or a scalar to be assigned directly to foo.