This is a very useful summary of these studies. One problem seems to be identifying the point at which usage becomes addition. I suspect there is a broad range in normal usage dependent on occupation and avocation,
As an IT person with some thirty years of Internet experience I have spent most of my days for many of those years online. There are many who do the same. While I use and evaluate internet technology so it is rare for me to be unconnected at any time I don’t have any particular anxiety related to that. I don’t like to be disconnected but that is based on years of being responsible for technical operations in corporations and in education. It’s my job and I enjoy it.
While I don’t doubt the reality of internet addiction I would think it is a good deal more complex than the simplest and most direct examples form the studies you cite. This is an old problem in addiction studies. One person can be considered and consider themselves to be normal but enjoy a glass of wine or two every evening. Another person may drink the same amount and consider themselves in need of recovery.
This is a complex thing controlled by individual characteristics and social standards.