Why are Kotlin interfaces "not open"?

When I type

open interface I

the Kotlin compiler warns me that modifier 'open' is redundant for 'interface'. This makes complete sense.

However, the reflection library seems to contradict this:

interface I
println(I::class.isOpen) // prints 'false'

How does this make sense? The KDoc of isOpen is very brief:

true if this class is open.

What exactly is the definition of "open" in Kotlin? I thought it meant "open to the possibility of being sub-typed by classes outside this file".