"What they are is x" — is singular "is" correct, and why?

Let's strip the non-essential elements from your construction (and add a bit of context to make it seem natural):

Dwarves? What they are is giants, not dwarves

Clever? What I am is mad, not clever.

(Notice that I have removed the inversion from your sentence, because it obscures the construction even more, while being non-essential.) This sounds correct to me, just as your example —except that the part before "is what" is pushed back a bit too far to my taste in your example, delaying the suspension a bit too long, which makes it a slightly awkward, but acceptable.

What seems to happen here is that a what subordinate clause acts as a singular subject of the third person, even if it refers to several objects semantically, and even if the subject complement (giants) is plural or of the first person. This is probably because our subconscious treats it as an abstract unit, which is by default a third person singular.

The same singular third person seems to be applied to which when it refers not to a specific word, but to a sentence or thought:

Achilles dragged Hector's body around the battlefield attached to his chariot. The Trojans were perplexed and enraged, which is just what Achilles wanted, or so it seemed.

You can see the third person singular being used to refer to the entire main clause (or to the thought expressed in it).

The main issue you're referring to is a case of inversion.

The first independent clause ends with the thought/ phrase "giants of a discipline," so for me it's quite natural to pick up from that and start the next independent clause with it:

...and giants in the world of scholarship is definitely what the authors of this volume are."

Also, I don't think the noun clause "what the authors of this volume are" can be replaced with another "simpler noun phrase" as it is a by-product of the inversion:

You're an angel. That's what you are.

If we simplify or "reinvert" it, we can't capture the same emphasis being expressed by the particular structure of the original.

P.S. I'm not sure what you meant by the "form of the copulative verb" in the title. Is it the subject and verb agreement between "giants in the world of..." and "is"? But I see nothing wrong with how all the "be" verbs are used in the quote.

UPDATE:

It seems you've completely moved the focus of your question by editing the title. But my answer still holds.

In the inverted structure, the subject of the sentence is the noun clause "what the authors of this volume are." And a noun clause as a subject takes a singular verb.

Ex.

How many fish there are doesn't matter.

What kind they are is important.