That conspiracy theory is more believable than anything that involves non-earth life forms coming to this planet. Just based on our understanding of physics, there’s no feasible way for aliens to visit us without some kind of generational ship. FTL maybe might possibly be doable, but our models don’t allow it with our current tech, and we haven’t seen any of the indications of trans-light travel that we’d expect to see based on our current knowledge.

All that to say, I believe in aliens on earth about as much as I believe in wizards and warlocks and shit.