It's complex because it's powerful. ~A is the most common directive; it simply prints an object. '~{...~}' (braces) iterates over a list, applying the directives within it to the list elements. Within that construct, '~^' exits if the last element of the list has been consumed; it's used here to keep the separating space from being printed after the last element. '~@(...~)' (parens) capitalizes words in the output of the directives it contains, downcasing the non-initial character of each word.
So there's a lot more power here than in 'printf' control strings. If you find the more exotic notation unintuitive -- and you certainly wouldn't be alone -- you don't have to use it.
It's disingenuous to claim a DSL is part of the language syntax. E.g. I would not consider the regex formatting specification as Python syntax. I mean, if we follow that line of thought, it's like saying Jinja is Python syntax.
If you're going to need to teach it for students to read and write programs in the wild than it's irrelevant whether it's 'part of the language syntax' or the ecosystem's syntax. 'Loop' is another one.
Lisp as an idea has a simple syntax but Common Lisp isn't even close to that platonic ideal.
So there's a lot more power here than in 'printf' control strings. If you find the more exotic notation unintuitive -- and you certainly wouldn't be alone -- you don't have to use it.