I agree with the sentiment, but I think your selling neurons short. They do grow, develop and change over time as a response to their inputs.
Just because a structure is larger and contains complex structures does not mean the larger system is more complex. For instance, I can create a logic gate with some friends. The logic gate is way simpler than the people composing it.
Neurons can have millions of mitochondria [1], which they dynamically move about the cell. Neurons are like amoeba with dozens of pseudopods when young. Amoeba are more complicated than worms, in terms of degrees of freedom, no?
Check this video of fetal neurons movin' about. So cute! https://youtu.be/hb7tjqhfDus
[1] https://www.frontiersin.org/articles/10.3389/fncir.2016.0002...
All a single neuron does is pass a signal to the synaptic cleft in response to a chemical or electrical signal at the other end. It's a simple subunit of a larger system.
Saying a single neuron is more complex than a worm is like saying a cat6 ethernet cable is more complex than a blast furnace.