Syntax is hierarchical, and not as much sequential as you may think. LLMs are based on these two facts.

I think your sentence would be much more accurate if you replace hierarchical with recursive.

True, at least conceptually (does brain actually do recursion?) - I was really thinking of linguist's hierarchical sentence parse trees, which are a nice visual representation of how different parts of sentence are syntactically unrelated and can therefore be processed in parallel, with levels of the "hierarchy" then corresponding to layers of the LLM.