I get where you're coming from, but I don't think they are saying to never use borders, but instead use them intentionally. They explain better than "Use less borders" below:
> Borders are a great way to distinguish two elements from one another, but using too many of them can make your design feel busy and cluttered
The example screenshot on the page shows a use case where they want to highlight a single row from a table, and using less borders is great for that. However, if they were trying to increase readability of the table overall, they may choose to keep the borders in place for contrast.
They used the same type of border over the first example, of course it doesn’t work. It’s “our genius design vs average nonexistent nonsense” type of move. Make table/list grid less pronounced, make the selection hint more saturated than a field bg, and now it looks better than #2.
Their selection looks like a hover hint in both cases as if they were afraid to tell a user that a row may have a selection on it.
> Borders are a great way to distinguish two elements from one another, but using too many of them can make your design feel busy and cluttered
The example screenshot on the page shows a use case where they want to highlight a single row from a table, and using less borders is great for that. However, if they were trying to increase readability of the table overall, they may choose to keep the borders in place for contrast.