When the iPhone was first released, it was rumoured that it could not be switched off; that when "off" it was still somewhat active and could be awakened remotely. This was just a rumour, never confirmed (to my knowledge) but not proven false either.
It is true that the "on/off" button is software: even when the phone is off, it still draws a tiny bit of current to detect when the button is pressed again. However, the CPU is powered down at that time (as can be shown by the fact that the phone can retain its battery life for weeks when "off", and it could not be that if its CPU was powered continuously). A dedicated circuit is used for that.
It can be said that a phone COULD be maliciously altered to be "somewhat on" when visibly "off", and could be activated remotely and/or briefly awaken once per hour to transmit some data. I am not aware of actual sightings of such modified phones, but in some circles (governments, big corporations) they require participants to strategic meetings to power down their phone and remove the battery; since iPhones don't support easy battery removal, there are "off boxes", electromagnetically shielded, to store the phones during the meeting. The "OFF Pocket" device that you link to is a one-phone version of such an "off box".
So such a device can be useful to counteract a potential threat which "makes sense" in some contexts (big businesses really spy on each other when they can pull it off), although there is no definite reported case of a modified smartphone turned into a spying device. Note that preventing communications is only half the job; the phone could still record voices and transmit the data when next linked to a network. In that sense, the really complete protection measure is not an "OFF Pocket" or an "off box", but really to store the phones in a separate room. The "OFF Pocket" device is an incomplete solution to a feared but unproven issue and, as such, falls into the same category as engraving a pentacle on the case to ward off evil spirits: it does more good to the user's mind than to the user's security.
(I suspect that the unofficial point of the "no battery / off box" system is to make people pay attention to the meeting, instead of looking at their emails.)