Why not 'elas'?
Just report. It is also correct.
Can't you also just leave it out and say "não são humanos"?
Yeah, why can't you omit the pronoun here?
Here we've a problem.
If you're talking to someone and you say "Não são humanos." the other person probably will ask you "Eles quem?" (Who?), but your sentence isn't wrong, sometimes we omit the pronoun because we already it.
Clarification please: They are not human is not the same as they are not humans. Is this lesson saying human must become plural when translated? Are there cases when it does not become plural?
It should always be pluralized when you have a plural sentence. Then, as we have no context, both "human" and "human" should be accepted here.