Observational and experimental are not opposed attributes. For astronomers the universe is the laboratory, and that is fine. An astronomer can say "I predict binary black holes to emit gravitational waves of a certain sort" and then go and find a binary black hole system and see if it does. That is experimentation.
I can possibly agree with that notion of experiment. My point, in the context of the discussion, was that just because we do not have a an idea for an experiment that would test string theory, in some definite way, does not exclude that there is some possible observation within reach.