Yeah they must've gotten into the mix from before. There's no way they can smell it from outside the sealed pack, and no reason they wouldn't be able to walk out of the pack after chewing their way in. Also you'd probably know if you had an infestation, considering that you must spend much of your time at home. Shitty, shitty brand!
I'm partial to Swiss Miss, myself. Variety pack. Keeps it interesting.
I buy generic & Swiss Miss hot cocoa mix, but I once bought the cheap stuff from my favorite dollar store. I didn't find any ants in the packets, but now glad I don't buy them anymore. I once returned soda (a name brand, too) that had a potato bug-type insect floating just under the bottle cap. I've also had stuff fly into my drink and die, and that sucks, since I still have to pour it out and get another.
It probably doesn't have to do with plant cleanliness, as much they handle/prevent an insect infestations. Cleanliness is part of it, but I once killed a line of ants walking across my clean kitchen floor, so maybe they don't inspect plants/equipment enough. Yuck, I probably wouldn't buy tea from them either...