Everything you ever wanted to know about the Leif Ericson kit can be found here...
AMT wanted to start a line of space opera type ships that would not be tied into any other property (so it could avoid license fees and whatnot) and hired Matt Jefferies to design it. Alas, the kit did not sell in the numbers AMT had hoped, so the plans for additional ships in the line were cancelled.
The kit was re-released in the early 70s labeled as the "Interplanetary UFO" and cast in "glow in the dark" plastic. The box art was updated to promote the GitD feature and it paralleled the appearance of the Trek kits' box art, leading to the natural assumption it was part of the Trek line.
I strongely suspect that was a calculated move by AMT to help sell the kit. Can't say that I blame them. Since Jefferies designed the ship, it "blended" easily with the Trek line.