FWIW, there was a BBC article about always-on audio detector apps. Describing an Android proof-of-concept app: "The battery drain during our experiments was minimal and, using wi-fi, there was no data plan spike."
Either way, it doesn't sound like that's what the article describes: they're talking about collecting and sending all audio wholesale. Sending that much audio data over 3g or LTE would be expensive (transcoding it to decrease payload would be expensive, too), and would surely be noticeable looking at data usage charts.
> using wi-fi, there was no data plan spike
Uh, yeah. Because it's using wifi. Phones are on wifi far less often than you'd imagine.
It's certainly possible, but it's just not plausible.
Having said that, quite a few people have their phones connected to wifi at home, which could mitigate these issues due to both less conspicuous data and power usage.