I think your last example is awkward, although I'd hesitate to say it's wrong.
I agree it's necessary to ask what "that" is referring to, but this is not an easy question to answer. It could refer to a group of things which are seen collectively as one group, or a scene/ensemble as JavaLatte has said, but there might be other possibilities. I've had a look online but can't find much except a rather obscure entry in the OED.
c. Used with a plural n. or numeral, instead of those: now only with
plurals treated as singulars (e.g. means, pains) or taken in a
collective sense.
Anyway, I don't have a grammar jargon answer to this, and I suspect it's a colloquial construction, and not formal. I should imagine in a formal piece, the writer would just avoid the problem and use different wording. However, the construction works regardless of the numbers involved (or the grammar).
Examples:
Is that one cup or two cups I see?
Is that two cups or four?
That's four cups, not two! (this usage is definitely colloquial)
Is that four zebras or five?
Is that two moons around Saturn I see? (if this is OK for Reuters, I'd say it's good enough for anyone).