@jeffmarc
It is not an organ - here is simplified explanation by AI what an organ is :

An organ is a specialized structure in living organisms that consists of multiple tissues working together to perform a specific function. It is a complex unit of the body that carries out essential physiological tasks.
Key Characteristics of Organs:

Composed of multiple tissue types
Performs a specific function
Contributes to the overall functioning of an organ system
Has a defined structure and location within the body

Examples of organs include the heart, lungs, liver, kidneys, and brain.
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We were thought that organ has its own blood supply (arteries and veins ), it's own singular function and very specific anatomy comprising of multiple cellular layers of differentiated tissue. For example heart is made of outside cover membrane layer named pericardium, than comes thick layer of muscle myocardium, than inside epithelial tissue endocardium. Hart has it's own innervation, it's own blood circulation etc etc. Very distinct "organ". Upon reading what is said on the net about lymphatic system it seems that somehow parts of lymphatic system they named "organs" . While spleen is an organ I have no idea how "nodes " came into play .
Nodes are part of lymphatic system like capillaries are part of circulatory system - we can not call capillaries "organs". This site explains the best lymphatic system and correctly calls nodes "structures" .
https://my.clevelandclinic.org/health/body/23131-lymph-nodes
But again, maybe USA textbooks use "organ" term more broadly, I can not attest to that.