Could Starlink really be used to fake an alien invasion?

That would be a very different job from providing communications. A satellite can reflect sunlight and appear as a moving point, but that is not the same as projecting a detailed image. The open sky is not a screen, and the thin air around the satellites does not offer an obvious surface for a giant picture.

So the claim would need evidence of some separate imaging system, not just the presence of satellites. The idea by itself does not show that such a system exists.

Seeing a string of lights or an unusual-looking object would not establish a staged event. Satellites, aircraft, and other ordinary things can look strange from the ground, especially when it is hard to judge distance or scale.

I would treat the projector theory as speculation unless there is independently checkable evidence for the mechanism. A satellite network being visible in the sky is not evidence of an alien-invasion plan.