The problem, precisely, is that if you assume that there is an underlying structure [noun] is raining [object], then from the very start you deny the possibility that the pronoun before the verb is in fact meaningless, i. e. a dummy word. If the pronoun refers to a noun and you can say what this noun is, then by definition it's not a dummy pronoun. My contention is that (maybe) the verb is really impersonal, i. e. its subject is truly a dummy word, not meant to mean anything but only appearing there to fill a syntactic slot.

The definition given at the start of the article is not sourced appropriately, I know; it just covers what I think are the features of what linguists call "dummy pronoun", based on my amateur experience on the matter. I know it's not completely wrong or misleading, though it probably needs a different form. The glossary definition above, OTOH, is a bit too vague, but in any case, note that the very first example of dummy word given below in the source is the it in It is raining.