The difference between hypotheses and theories isn't about whether one is more likely true, they are different kinds of structures with different roles.
A hypothesis is ideally 'atomic': it's making a single assertion that should be falsifiable by experiment.
A theory provides a specific way of modeling the elements of some field of study, and how those elements interact with one another.
It should be structured in such a way that it can describe the components of any experiment that might be set up to confirm/reject a hypothesis in this field.
And once an experiment is described in the terms of some theory, the theory should provide a systematic way of calculating what the result of the experiment should be.
That's where the connection between hypotheses and theory comes in: actually conducting experiments assigns either Acceptance or Rejection to a set of hypothesis; ideally, the theory assigns the same Accept/Reject values to the same hypotheses in its predictions (this is probably never 100% the case in practice though—esp. not in an active field of study).
The structure of String Theory is much closer to theory than hypothesis. (It may have limitations in how much it models physical 'experiments,' but it's at least much more of a 'system description' than an atomic assertion.)
Physicists and philosophers of science give those terms different
meanings. E.g. what a philosopher of science would call a theory is
called model by physicists. What physicists call a theory is more
a broad framework in which more concrete models can be build.
From the view of a philosopher of science string theory isn’t even a
hypothesis as a hypothesis needs to be testable
