For some context, I've been programming in Go for about six years now, and I've literally never even thought about this.

In general, you really haven't got much reason to be worried about something's "interface type", because if you're really worried about something's "type" (which can happen) you are almost certainly worried about it's "concrete type". The only interesting thing you can ask about the interface type directly would be "Does the value inside this interface implement this interface?", and the answer is statically yes, the compiler assured that, and the interface type is statically assured at runtime, so it's not a very interesting question. Even if fmt added a way to "see" the interface type passed in, I can't imagine what it would be useful for, because it could only ever be:

     var x SomeInterfaceType

     // some number of lines of code

     fmt.Printf("%I\n", x)

which would statically print "SomeInterfaceType".

So, I mean, it's not like anything said in that blog post is false necessarily, but if you're collecting reasons to hate on Go's nil handling, there's no real justification in adding this to your list. I'd also add that I answer newbie questions a lot on Reddit, and nothing even remotely like this has ever come up there, either.