I think the translator may have misunderstood this sentence: "An explicit index inside an array is also considered a container."
I will now try to explain the intended meaning of the sentence.
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.