It's not clear which kind of nil interface you mean. :)

In Go, interface variables are a pair of values: a pointer to the concrete value stored in the variable, and a pointer to the type of that concrete value (i.e. the actual struct).

So there are essentially three kinds of nil in Go: a raw nil pointer, an interface variable with both nil value and type pointers, and an interface variable with a nil value pointer, but a non-nil type pointer.

One way that the two "interface nils" differ is that, to follow the article's example, if you have a Cat variable containing a nil *Tabby, you can invoke Meow on it, and it'll print meow. This works because Tabby's Meow doesn't try to dereference its pointer-to-Tabby. However, if you have a Cat variable that's completely empty, invoking Meow on it will fail.