When we are locked in by the Kessler Syndrome, are the extra terresterial aliens then also locked out from trying to «visit» us? ;)
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If you're travelling between stars, you do actually need star trek sci-fi shields. The relative velocity turns interstellar dust into bullets that would obliterate an unshielded ship. Assuming those shields can operate in proximity to a planet, they could simply shove their way through the debris cloud.
Alternately, an interstellar ship probably has a lot of power to throw around and likely a lot of time they're willing to spend. It's not unreasonable to think that a starship would be equipped to deal with some amount of orbital debris. Probably point defense lasers or something. Maybe a tractor beam.