In Clojure (and I’m pretty sure Scheme), ! and ? are not sigils in the sense that they’re special syntax like, say, @ (which is a reader macro), or @ and @@ in Ruby: they just tend to be used by convention. ! and ? are no different than a or b.
They somewhat follow the OP's definition of sigil (in the yellow box), which doesn't say that sigils have to be interpreted in a special way, only that they convey some meaning to the programmer (although it's a bit unclear - communicate to whom, programmer or interpreter?). They are also not at the beginning of the word, but that seems to be rather arbitrary requirement.
You can read more about the Clojure reader here: https://clojure.org/reference/reader