I wouldn’t neccesarily say that UFO’S are Alien’s.
I’d also say they could be Time Travellers or people from Parallel Universes. Both of these are scientifically possible.
In both those scenarios vast distances wouldn’t have to be navigated, and with time travel we’ve got the whole of the Earth’s lifespan to invent it. If Time Travellers at any point in the future wanted to come back to the early 21st century for a look around they could be identified as UFO’S. Why not?