It has to take place right after Chi Ceti. It is the only way I see it making sense.
Now there are two possible explanations at least the way I see it.
Having a whole planet getting burned down isn't a very good thing, so just announcing this wouldn't be the best course of action. So I imagine that the higher command people surely know of the Covenant, but it could be possible that they are withholding this indo from the lowly recruits until they know more about it.
Or a much simpler explanantion, the Covenant blocked communications of the planet.
Also John is wearing Mk IV armor though the creators of the package are stupid and made it look just like Mk VI armor. Anyways this means that Chief has gone through the battle of Chi Ceti, so in while they Soartans are doing whatever they somehow find out about the Covenant at whereever and head over there.