It's outside of the type system for two-ish reasons. First, because of the point the sibling comment made: while you can assign the singleton value `null` to any reference type's binding (but not primitives, so it's not even the bottom value for all types), it's not actually a value of that type, bottom or not. E.g, I can assign `null` to a `String` binding, or pass it as a `String` parameter, but I can't actually get it to behave as a `String`, so any call to a `String` method results in a runtime error.
That leads to the second reason I say it's "outside the type system": you can't explicitly write down the `null` type in the language. The reason you get a runtime error when we pretend like the `null` value is a `String` is because it's not a `String`- rather, the binding actually has the type `String | null`, but the compiler allows (forces us) to pretend that the binding is a `String`. Since we can't describe nullability in the written Java language (express it in the type system's language), but it does exist at runtime, I say it's outside the type system.
That leads to the second reason I say it's "outside the type system": you can't explicitly write down the `null` type in the language. The reason you get a runtime error when we pretend like the `null` value is a `String` is because it's not a `String`- rather, the binding actually has the type `String | null`, but the compiler allows (forces us) to pretend that the binding is a `String`. Since we can't describe nullability in the written Java language (express it in the type system's language), but it does exist at runtime, I say it's outside the type system.