Should be called string hypothesis.
Unfortunately, a fully fleshed out theory with centuries of matching experimental data is still called a theory.
Even in mathematics, there are different definitions of "theory". One is that a theory is a body of knowledge, like set theory or automata theory. This is where I would group string theory as well.
Another definition from mathematical logic is that a theory is a set of axioms and deduction rules. Fascinatingly, those have in common with scientific theories that they can (if sufficiently complex) only ever be falsified, not verified.