Not having read the whole article, you do not understand the context of what was said. Suffice it to say only having one rifle, its own er has learned to use it in whatever situation.
The point is that the far, far, far, far more common scenario today is that the shooter who has one rifle has not learned to use it especially well at all. Most people today who own only one gun are casual shooters. Lots of families have an old shotgun or rifle that just sits in a closet until it is drug out thrice a decade to plink a few cans or flail away in a friend's field at some pretty-safe doves.
Were things different for a generation that grew up during the depression or in a rural, cash-poor/manufactured-goods-scarce environment? Sure, although I doubt having multiple guns has ever been correlated overall with a lack of skill (certainly it was and is possible for unskilled individuals to acquire multiple guns - but having more than one gun is not a good indicator that a person doesn't know much about guns or how to use them). But today? Nah, the saying is upside down.
Not being a motorcycle rider myself, I express no opinion on the "biker" question. But I would venture that most competitive motorcycle racers probably have more than one bike, whereas a solid majority of the more incompetent riders own only one - probably their first.