OK, the rule of "des" becoming "de" is when the noun is an object:
- il dit des mots -- il ne dit pas de mots.
In this case, with verb "être", the function of "mots" in the sentence is not that of an object, but of an attribute.
So "des" is simply the plural of "un":
- c'est un mot (affirmative, singular) -- ce n'est pas un mot (negative, singular)
- ce sont des mots (affirmative, singular) -- ce ne sont pas des mots (negative, plural)
Look at the singular: "ce n'est pas un mot"
now the plural: "ce ne sont pas des mots" - "des" is just the plural of "un"
Now add an adjective: "ce ne sont pas de vains mots" - "des" has become "de" because of the adjective.
Now let's make it positive: "ce sont des mots", then "ce sont de vains mots".
You can see now that the fact that the sentence is negative has no effect on the article.
To answer the query that 'these' is the object. Unfortunately not. The sentence has no object. In sentences like these, conjugated with 'to be', what looks like the object (mots) I guess are called 'attributes' in french, but in english I was taught they were called 'subjective completions'.