The first answer is effectively correct, but if you want something more technical, remember that mutating is equivalent to inout on self, and inout is formally “copy-in, copy-out”. So the Law of Exclusivity says that f is being accessed for the entire duration of the call to modifyX. But within modifyX, self is an independent value that was initialized from f. Then that value is accessed for the duration of the call to modifyY, being copied in to another self.
The way you get an exclusivity violation on one thread is by referring back to the original storage, which can only happen if it’s a global or if there’s a reference type involved (class or closure).