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
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).
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).