To understand what it might mean, we need to consider:
* What is being referred to as "them"? What specific group or object are we talking about?
* What is the context of this statement? What is the larger conversation or situation where this phrase is being used?
Possible Interpretations:
* Lack of a physical characteristic: It could mean that something lacks a specific physical feature that would make it "short". For example, "These buildings are not short because they don't have the low ceilings that make them short."
* Lack of a defining quality: It could mean that something lacks a specific quality that would define it as "short". For example, "The movie is not short because it doesn't have the fast pacing that makes it short."
* A metaphorical interpretation: It could be a metaphorical expression, where "short" is used to represent something else, like a lack of time or resources. For example, "They are not short on talent, because they have the creativity that makes them great."
To get a clearer meaning, please provide more context or clarify the sentence.