Is "that's two words" grammatically correct?

First, let's expand the contraction to clearly show the subject and the verb:

That is two words.

"That"--demonstrative pronoun (singular) [https://en.wikipedia.org/wiki/That ]

"is"--a linking verb (singular; NOT an action verb).

When a linking verb is used

"A linking verb ("is," "are," "was," "were," "seem" and others) agrees with its subject, not its complement."

[ https://writing.wisc.edu/handbook/grammarpunct/subjectverb/ ]

"words"--subject complement (quantified by "two"--numeral determiner)

"A subject complement follows a linking verb; it is normally an adjective or a noun that renames or defines in some way the subject."

[http://grammar.ccc.commnet.edu/grammar/objects.htm ]

Based on that, I don't think it would've mattered if there were 1000 words, as long as "that" referred to something singular, such as an essay...or, another example, a Stephen King novel: I read Duma Key. That's 609 pages!

The last page is two sentences:

Know when you're finished, and when you are, put your pencil or your paintbrush down. All the rest is only life.

(Which reminds me, something singular, as I put it, may be a mass noun, not necessarily a countable noun, if I'm not mistaken...but I'm finished now.)