Bit of context: In a documentary a guy finds two bears he raised the year before; at the beginning he doesn't recognize them but eventually he shouts, "It has to be them!"
To my Spanish ears the correct sentence would have been "They have to be them" and Google Translate kind of backs me but still find contradictory answers to this.
If we see two things, we might refer to what we are looking on as a single scene (of two things) or as two things.
"The thing I am seeing has to be the two bears I raised." → "It has to be them."
"The two bears I am seeing have to be the two bears I raised." → "They have to be them."
Both are therefore fine.
There is also a very common placeholder use of it used "with reference to an abstract thing, or a matter expressed or implied in a statement, or occupying the attention of the speaker."
And here it can be used with anything that we later give more information about, whether that thing "contains" a plurality or not.
"It has to be them" is current usage. "They have to be them", on the other hand, sounds awkward.
"It has to be them," "It must be them," "They have to be them," are all correct to my ears. I think there is a slight discord with "They have to be them" because of the underlying idea: they are them. "They are them" is quite unidiomatic, whereas the extension you suggest is diluted and so (somewhat) tolerable.
"They have to be them" would be the logical answer, but it is a little like saying "I am me" which can only be true, and slightly mad...
Imagine instead the French: "Il faut que c'est eux!"
The "Il faut" means "it must be the case", so the sentence really means "It must be the case that it is them!" However, this is a mouthful, so it gets trimmed.
"It has to be them!"
In British English, "It's got to be them!" is about the most idiomatic way I can think of to express this idea.
