This study refers to CPR in case of cardiac arrest only! You should still apply the 30:2 rule to patients that drowned, have airway obstructions or drug overdoses.
In any case, when you approach a patient, most of the time you will not know what the cause of collapse is, unless it is witnessed (someone choking in a lunch room, someone grabbing their chest in case of heart attack). So in this case you have to check for a good airway, by listening and feeling and sometimes by giving a breath mouth-to-mouth to see if the chest rises)

In any case, for people with first aid training i suggest carrying some kind of barrier device like a pocket mask to perform CPR. Some of the smaller device are really cheap and small (fit on your key ring).
I always carry my pocket mask and gloves with me, but then I am also obligated by law to help people because of my first aid ticket, and i want to play it safe.

Also, when i was tought in first aid class, we were specifically instructed never to give mouth-to-mouth unprotected, because the first rule in first aid is to watch out for your own safety!