While this behavior can be counterintuitive, it's easy to understand once you have the right mental model of interface values. A nil interface is an empty interface. Once p is assigned to i, then i is no longer empty: it holds a nil pointer to an int. Whether the interface is empty or not is one question, answered by comparing it against nil. Whether the value the interface holds is a nil value is another question, answered by downcasting and then checking the resulting value.

I don't blame people for being confused by this the first time they encounter it. However, anyone who finds it persistently challenging to understand is not going to have much luck with the subtler language features of C++, Rust, Typescript, etc.