It's a bit of a tricky one, as "knot" usually refers to a tangle of string or a fastening. However, in this context, it can be used metaphorically to mean something that is real and concrete, not imagined or made up.
For example:
* "The story was a knot of truth, not a fabrication."
This sentence uses "knot" to imply that the story was based on real events, not made up.