For one you can read the article. But it's a long standing thing. When you license a film or show for streaming, you don't inherently get the right to show ads during it. Some studios specifically exclude the right, some charge more, some don't care. The deals made for some of the Netflix originals don't even allow it, so they can't show their "own" shows with ads. The reality is often times streaming services aren't even making their original shows, they're just paying a bunch of money for exclusive distribution in one or more markets. So, older deals especially, Netflix didn't negotiate for ad rights because they weren't showing ads back then. Some production companies don't want ads running during their prestige show.
"Netflix Originals" are content that is produced, co-produced, or distributed exclusively by Netflix. Note that last type. Anything they have exclusive distribution rights for can be labeled an original, even things they didn't make.