What the comment you are replying to said was that it is a mathematical formalism and not a physical model, and that they are still working to construct a physical model. So, it is not a physical theory, but the only people trained in it were first trained as physicists, and are employed by physics departments, and everything in it is constrained to fit in a physical theory.
The situation is far from ideal; there is no other plausible candidate for a physical theory compatible with gravitation and quantum field theory. But at the same time, we have literally no means to measure any case where they interact meaningfully. We can measure a fountain of cold neutrons falling in a gravitational field, but only the extremely weak field we are in. So, while it is a problem that nobody can devise a test for a physical string theory, nobody can devise a test for any other theory that would seek to fill the role.
The situation is far from ideal; there is no other plausible candidate for a physical theory compatible with gravitation and quantum field theory. But at the same time, we have literally no means to measure any case where they interact meaningfully. We can measure a fountain of cold neutrons falling in a gravitational field, but only the extremely weak field we are in. So, while it is a problem that nobody can devise a test for a physical string theory, nobody can devise a test for any other theory that would seek to fill the role.