As I understand it, thereer are at least 2 and probably or evidently 3-4 ways around the speed of light 'problem'
ET's tend to laugh at folks who ask them about that. As though they were talking to folks not yet in kindergarten.
1) Technological ways to bend space/time evidently exist as a way around the speed of light issue.
2) For lack of a better word--using other dimensions as a short cut between two very distant points in our dimension is another way around the speed of light 'problem.'
I don't recall the others but I didn't understand them well anyway.
OK, 2 things:
Way off on the light year travel to Alpha Centauri: it is something like 4.36 light years from Earth.
But, oddly enough, that is kind of a distinction without a difference, because now, the only way for ET life to exist is for them to (1) travel the speed of light (2) and travel through some time space portal.
Having said that, you could pretty much make the statement that we cannot have been visited by ET/UFO's unless they (1) traveled the speed of light, and (2) by traveling speed of light, were able to make use of some time space portal.
OK, here's the deal, it is outlandish conjecture, I mean it is just a fantasy to say that those two things exist, or can exist. No one has ever defended a thesis or disseration on those two topics, nor any credible peer reviewed article.
So, I comeback to the position that Fermi first posed: Where did they come from?? Also, if they had gone to such a great length to come to this planet, I don't thing they would have been overly worried about the politeness of barging in on earth; obviously a scientific intelligence would have made no qualms about setting down, collecting life samples, making contact, exchanging ideas.
TO me it is just an argument that conclusively dispels the idea until someone can prove (1) travel at the speed of light is possible, or (2) some time space portal allows such travel. Until then, there is no possibility that we could have been visited.