Unless it's been firmed up a great deal in recent times.
Predicts that special relativity holds up at all scales (check, according to all evidence so far), predicts general relativity at low energy scales (check).
So it's false that it has no testable predictions. None of this happened "in recent times" though, it's been understood for a long time.
Imagine if someone had said 30 years ago that the "higgs boson theory" is a failure because we couldn't then perform the experiments to detect it.
Don't get me wrong, I still regard string theory as a big success. It taught us a lot about mathematics and field theories in the last decades. However the predictive nature is basically non-existant so far.
In addition you haven't addressed the main point here which is that when some people say "can predict" they mean "in principle it can predict" whereas other mean "can predict today with currently available technological means". Regarding the former: yes it can. I already gave the example of one such prediction, but here's another one: all particles niches have stringy modes in their spectra. Regarding the later: maybe, but thats a problem with our technology, not with the theory.
I feel this conservation is going in circles already.
So you made your prediction only by choosing an axiom.
There is a reason that even some proponents of string theory call it the theory of anything.