Yeah, I realized that the moment after I posted it. Derp moment. But it makes you wonder why they were going back there to destroy the planet. Why bother teaching humans language if you're just going to kill them? And why point them to a constellation so far away that they couldn't even see it with a telescope?
But more interesting to me are the following two conundrums.
1) If David spent two years deciphering ancient languages to their roots, does that mean he spent the two years on the ship during its travel to decipher the languages? Why wouldn't they have left after David was sure that he had deciphered their language? What if everyone awakens from hypersleep and David said, "I'm sorry, I just couldn't figure out their language. I can't say with any certainty that I can communicate with them. Given the records we have, I would have to say that I couldn't even derive their language in twenty years." Then what? They were all just hoping that he'd finish in time? To which you might say, "Well Olde, they never said which two years he spent decoding it. Maybe it was the two years before they started the journey, which they began once he was finished." Which leads me to my second point...
2) Just think about this for a minute. David decodes a primal, original language derived from commonalities between ancient languages. Just look at how problematic that is. Not only are many ancient languages completely different, but we don't even have an oral record of anything before about a century ago. How on Earth does David know what civilisations two, three, five, ten, twenty, or thirty or more centuries ago sounded like? How could he construct a language out of what is essentially an absent record? Sure there are some records here and there, but oftentimes we don't even know how certain written languages sounded. Yet David casually deconstructs their language, all while making time to play basketball while riding a bicycle, watching Lawrence of Arabia, and combing his hair. What is he, the world's greatest paleolinguistic supercomputer? Does he knock that task out in a week or something? And why would something with terrabytes of linguistic data, accomplishing one of humanity's greatest achievements, first of all be interested in anything other than language (I don't think he mentions any human language once during Prometheus) but also be tasked with regular maintenance work and just exist for servitude in getting people their vodka and robes? And why wouldn't he be interested in the Engineers' language in Covenant--why does he use them to experiment on creating organic life when he's clearly programmed with this ingenious linguistic supercomputer functionality?