For context, the example from the Serde wiki written in Java using Jackson

  public record Point(int x, int y) {}

  // [...]

  var objectMapper = new ObjectMapper();
  var jsonString = objectMapper.writeValueAsString(new Point(2,1));

It isn't obvious to me what advantage Serde offers that you couldn't get with Jackson or other similar libraries in Java (although I get that Java and Rust are different languages, and that there may not be something this ergonomic in the likes of C++)

For that example there's no advantage. But it's really, really easy to find examples where there are (the simplest example is, anything with pointers with @NonNull annotations, or anything which is performance-sensitive and contains other objects).