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In most cases they are using dotted horizontal line which is better than a solid line.

In Google Slides or Excel it's easy to use low-contrast shading between dark and light backgrounds which is another less-intrusive way to delineate rows without using a solid line.

This is an annoying trend; it's true that row lines add a lot of noise that distracts from the content, but you should use color and value to reduce the dominance of the table lines, not remove them altogether.

I think it depends on the width of the table.

For a small 3 column table, it is not necessary. The tables on the page "Example Tables I" could have done without horizontal lines and a little more space between rows would have been enough.

Hmmm...I had the opposite reaction. "Good guidelines, why don't their Economist examples follow them?" I find very light shading to be way better than horizontal lines.