Or the aliens come back years later and say "NOOOOO! The instructions call for sarson stone!"

Then the alien next to him leans in and whispers in his ear. He looks at the blueprints, whispers something back, then says:

"Apologies, humans, we seem to have translated the material incorrectly, you will have to start over and make this all out of metal."

The other alien whispers again.

"Once again, we apologize, but it turns out we have also landed in the wrong century. We are on our way to the 20th century. Again, sorry this happened. We will be sending holograms of religious iconography to hopefully cause discord and you will exterminate yourselves so nobody hears of this. Good day."