So adding the words "on tatooine" to the end of that report would help that a lot. Now, even though they have no clue wtf mos eisley is maybe they heard of tatooine at least in passing.
I think you're over-analysing - the words spoken in a film aren't for the benefit of the characters, they're for the benefit of the audience. We know where Mos Eisley is - we've just seen it. And now we know that Tarkin knows this ship came from that same place. I think it's safe to assume that Tarkin would have been kept informed of the Tantive IV's movements and subsequent events, anyway.
Even if adding "on Tatooine" might be more correct, that doesn't always make something [i]right[/] - kinda like how people on TV put the phone down without saying "bye", don't lock their cars when they leave them on the street, or are terrible liars but always get away with it.
Not to mention the fact that Tatooine isn't actually named in ANH, which would make an inserted mention a bit of an orphan.
DE