In the notes it mentions that a noun without a classifier can be "Implying [a] characteristic of that noun as a whole. Demonstrating a truth/fact about that noun."
Interesting. The plural lesson states 'Important: những and các are to demonstrate plurality only. They are not equivalent to "some", "a few", "many".' My understanding was that the indicator was the equivalent of the English plural 's'. Is there more to it than that?
Think of it this way:
You and I are humans. Here, individually each of us is a human.
We are human. Here we're talking about the human race, and it's no "s".
"Họ thích động vật" is the same way as "human." It refers to the whole animal kingdom. If you go to the zoo and you say "I like THESE/THOSE animals," then you would need to use những or các to demonstrate plurality. Does that make sense?
Yeah that's what I thought after I put in (They like the animal) and said it was wrong.