Because when the compiler sees:
@var = some_expression
the compiler will try to infer the type from some_expression. In this case it’s this:
x || nil
Can we infer something from x? Let’s look if it has a type restriction. Nope, it doesn’t have any. Can we infer something from nil? Yes! It’s nil so it must be of type Nil. Well, Nil or something else (x) that we don’t know the type of. So the end result is that we can only infer it to be Nil.
It doesn’t matter what you pass to the constructor. The compiler doesn’t look at calls to infer the type of instance vars: just what you assign to @x inside the constructor (or, well, any other method, but never calls to methods.)