Das is plural?
In the phrase "Das sind meine Katzen", what is the rationale for using "Das"? Looking through tables of pronouns and the site's drop-down translations, I don't see any suggestion that "Das" should be used for plural nouns.
Question becomes insight: I now realize this is like answering the question, "What is making that godawful noise?" with, "It's/That's my cats." The pronoun "Das" isn't really referring to "cats", but to the abstract concept of the unknown thing making the noise. If you said, "Die sind meine Katzen," that would be equivalent to saying, "They are my cats," which is different from the above, and also from "These/those are my cats."
I like that this construction implies bewildering ignorance on the part of the person you're telling about your cats.
In this case das is NOT an article. Other than having function of neuter article it also means this/that. Even if it was an article it would stand next to noun. So translation of that sentence is: "These are my cats". Pay attention to declension of pronoun "mein" (my). You use MEINE Katzen because Katzen is plural. Article isn't necessary because of the pronoun. It's same as in English. You would say: 'These are my cats' not 'These are the my cats". I hope you understand it.
And you will find this construction used by native German speakers on the web. "Das sind meine Katzen" may not be logical, but it is idiomatic, that is, it is an expression that German speakers actually say and that they actually write.
"When it is used with the verb sein, the form das can refer to singular and plural nouns of all three genders"