On thing he did not go into in the article is how you tip in a cashless society (maybe people don't tip in Sweden, but they do in the U.S.). My friend Phil the waiter explained to me that if you tip in cash, the waiter gets the money immediately. If you add the tip to your bill and then pay with a credit card, the waiter has to wait for his paycheck to get the tip.
As a former longtime waitress whose two sons have often worked as tipped wait assistants (busboys/food runners), I can tell you that's not always, or maybe not even usually, how it works.
When I was a waitress many centuries ago, your base pay was less than minimum wage and you had to declare to the employer enough tips to equal minimum wage. The actual tips were far beyond that, you got them in cash regardless of the customer's payment form, and walked out with a pocketful of money every night. I would be guilty of tax evasion, I guess, except that I didn't make enough money back in them days to pay taxes anyway, so whatever I paid I would get refunded the following year.
At the restaurants where my sons have worked over the past few years (probably about six or so establishments between them), their tips come from the waiters. The waiters are expected, though not forced or monitored, to pay the wait assistants a percentage of their tips. Sometimes the servers hand them cash directly, sometimes it gets funneled through the restaurants and they receive it in their paychecks. Then they not only have to wait for it, but a larger percentage gets withheld for taxes. In their case, it's not a huge deal, because again they don't make enough in a year to pay taxes at all so they get bigger refunds the following spring. But for obvious reasons, they're not fond of that system. Another problem with it is the one you hinted at -- the paycheck system means the waiters tip anonymously, so those who undertip aren't identifiable.
In any case, none of it -- in the restaurants where they've worked -- has had anything to do with what form of payment the customer uses.
My understanding is that tipping is unusual throughout Europe, but correct me if I'm wrong, Eurobrokies!