Any idea why he gets so much pushback, when string theorists get a pass? Is it because "alien tech" is more easy to understand as a concept than Calabi-Yau manifolds?

because of UFO Conspiracy Theorists. When someone says Alien in a serious context, most people immediately associate it with UFO nutjobs.

String theory has not really made its debut in the conspiracy crowd afaik. I think "Quantum-___" has done so, especially with the "collapse of the wavefunction through the observer" it has so many esoteric people raving.

String theory is so meaningless to the normal person.

I think it comes from a place of insecurity. people get sensitive about it because all astronomy is pointless and arbitrary, so someone having outlandish fun while doing it runs the risk of highlighting this fact.