Not taking either side here - but surely a lot of cosmological phenomena couldn't be testable/verifiable at the time they were theorized either?
(thinking of black holes for example - they were theorized way before we had observations. And presumably a lot of particle physics can similarly be theorized before we built the technology to experimentally verify them)
Black holes were theorized and you could design experiments that, given the proper instruments, would allow them to be detected. Relativity was similar (famously, the curvature of spacetime was demonstrated during a solar eclipse by being able to see stars that should have been behind the sun).
String theory has nothing even theorized that would allow us to prove it.