Do keep in mind that the article, which I believe I was able to read somewhere else, is talking about a relatively new class of seating. For international flights, airlines had gravitated to a premium class, a "plus" type economy, which was basically the same seat as economy, just more room, and main cabin. Most had done away with a distinction between First and Business classes long ago, I believe of the big 3, only AA still has something they sell as "Business".
The airlines then moved premium class to the lay-flat seating, and even compartments; leaving a big gap between "Premium" and "Plus", so in comes "Premium Economy".
Delta only introduced this in the last year or so, calling it Premium Select, so on a trans-Atlantic delta flight, you will find Delta One, the lay flat option that takes up lots of real estate, Premium Select, Comfort Plus, and Main Cabin. Premium select uses a little wider seat, more leg room, and the seat is more like a recliner, having a foot support. They added a larger entertainment screen, but most everything else is main cabin amenities. It is a bit like bring back "business" class, but geared for travelers that want some comfort, but do not want to shell out to lay flat. Bonus for the airline is that they can sell a premium ticket without taking up much more space than a main cabin seat. Cost point wise, Plus runs about 1/3 more than main, Premium is double main, and Delta One 3-4 time main. Main also sees discounts periodically, for Premium and Delta One, prices are much firmer.
Even though it is tempting, I have not tried Premium Select. I am cheap, Main Cabin is fine, and they nearly always upgrade me to Comfort Plus immediately. I know many on here will not fly less than First Class, I guess I would prefer to spend that money on meals and things in Europe.
An Edit: If you have not flown Delta in the last year or two...you know, the Covid thing...by adding Premium Select, that pushes Comfort Plus back, so now Comfort Plus is basically over the wing...not a big deal, but a bit frustrating for a window person like me who likes to see the scenery.