I'm assuming he meant why would they expose or promote the use of a construct that bypasses type safety in a statically typed language. Although `interface {}` is not some sort of special feature, it's still circumventing the type system.
It however does pass the type checking to run-time instead of compile-time, which is a trade-off that some feel is worth the language simplicity.
That's absolutely still loosing type safety. Reflections don't substitute type safety.
I also wouldn't consider Go type assertions to be in the same class as reflection. A type assertion is the way you extract and check the underlying type from an interface, and is quite common even when using more complex interfaces. There's also no way to make use of the underlying type from an interface without first asserting it as that type.