whenever I have questions like this I type up a little application to
test out what the compiler does. Then I never have to guess!!
This program compiles fine:
so I would suggest that website is wrong.
Interfaces are implicitly abstract, as are the methods in an interface; as you mentioned, it's only logical since they can't implement behavior in the interface.
Note that use of the "abstract" modifier here is considered obsolete and should not be used explicitly. (JLS 9.1.1.1)
Rob