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?

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.

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.