There's an answer for this in the notes to this lesson. It says "Single-syllable neuter nouns, such as hus 'house' and dyr 'animal', often do not change spelling in the indefinite plural. How then can you tell the difference between hus meaning house and hus meaning houses? That depends on context and adjective endings, which we will cover a bit later in the course."
In this sentence, it seems to me that if it were singular it would be either dyret 'the animal' or et dyr 'an animal'. Since it is neither, by process of elimination it must be plural 'animals'. At least that's how I think of it, but I'm only a beginner myself so I might not have it exactly right.
I suppose it's an exception to the one-syllable rule? As I said, I'm only a beginner. But you can still tell by the context here. If it were meant to be 'a sandwich' it would say 'et smørbrød', and if it were meant to be 'the sandwich' it would say 'smørbrødet'. It doesn't make sense to say 'Animals don't eat sandwich' so that option is also out. 'Animals don't eat sandwiches' is the only possibility left and must be the intended meaning.
I think because it sounds a bit odd without an article.
"The/those animals are not eating sandwiches" for example sounds fine, but without the article you're making a present-tense continuous statement about animals as a category, rather than any animals in particular, and I think that's where it falls down.