When I was on one, in the exact same situation, it was impossibile not to think about the staggering quantity of humans in this world, each of them separated by language, culture, beliefs, needs, space. Suddenly, communication and building bridges becomes one of the really important questions.
Lots of questions, but I've never heard this development phrased in a positive way before. It's usually associated with social withdrawal and depression.
Saying not talking to a coworker over lunch is rude seems harsh and rude.
What I understood from the OP was something like "people who share lunch and instead of interacting prefer to constantly check their phones for FB, news, interesting links and so on".
If this is the subject, then my take is: doing this "in presence of others is rude". It is much better to sit together with others even without speaking, or not adding much to the conversation in any case.
Sitting alone to read stuff over your smartphone while eating (also: substitute paper book if you prefer) is not the most socially inviting move but it is way better than sitting together with someone else and ignoring them.
Assuming we agree on what I wrote above, I will not go back to my original point: if when you are together with others you feel like you would prefer be alone and checking out something on Wikipedia (or try to find excuses to somehow glimpse at you FB feed) it could just be a symptom of not really being interested in Frank's son antics at the kindergarten or Jane's rant about the current POTUS or the last football's match.
If this happens also with relatives, this is perfectly normal, too: you have probably just different interests (compounded by the fact that if you have known each other for decades there is a high risk of getting mostly rehashes of stuff you know already).
What I am trying to get across is: "if you spend time with people that you consider genuinely interesting, I am pretty sure that the smartphone will stay in your pocket (unless if someone wants to check something on wikipedia or whatever)".
Note also that (to more specifically address what you wrote) it is perfectly ok to share a meal with little to no interaction (apart from "please pass the salt" or whatever). It is in fact a proof that you are perfectly comfortable as a couple or a group, but again, this is different from one or more participants focusing on their own small screens as if they were completely alone at the table.
And the David Foster Wallace quote you requoted was insightful: "Lonely people tend, rather, to be lonely because they decline to bear the psychic costs of being around other humans. They are allergic to people. People affect them too strongly."
You write near the end of your review: "The book clarified for me an important point, though: Solitude is something that we actively seek, while Loneliness is something that we try to escape. Apart from that, they are exactly the same thing: being alone (which in turn is something that happens quite often - in fact, humans are "alone" most of the time)."
Given that, what are your thoughts then on the phrase "alone in a crowd"? For example: https://www.psychologytoday.com/us/blog/the-introverts-corne... "We know loneliness may or may not be related to being alone. Introverts can be perfectly happy alone, or terribly lonely in a crowd. But if introverts are at any particular risk for loneliness, it could be because we set a high bar for friendship. We desire and require deep connections and would rather be lonely alone than in a crowd. But realistically, those deep connections are not easy to find, and if we get caught short and our only choice is superficial socializing or nothing, we can get lonely."
Also, you say the book suggests: "Another of the prevalent journalism theories (internet and social media will result in an increase of individualism and loneliness) seems not to be supported by empirical studies: those who use social media report having a more active social life and more frequent personal interaction with other (outside of social media itself, of course)."
I wonder about that, given the notion of narrowly focused "supernormal stimuli" and how super-fascinating (anti-)social media can potentially displace more holistic human interactions especially over meals. But websites that arrange meetings of people with common interests can counter that trend, so obviously that's not a black and white thing. A related book on that if you are looking for something to summarize next: https://en.wikipedia.org/wiki/Supernormal_Stimuli