Nope, the string path integral is not about free strings only. It allows to compute all string amplitudes in perturbatuon theory. This, by the way, one of simple predicrions of string theory, together wirh very peculiar spectrum of states (too bad both can be distinguished from the corresponding effective QFT at too high energies to actualy verify the predictions).
The QFT counterpart of that is the worldline formalism, not
Both, of coursez fail in highly non-perturbative regimes. But making the theory work non-perturbatively is part of making sense of it.
So basically string theory has no principles, postulates, predictions, nor equations.
That's not what that post said.