In the early days of Atari's 8-bit machines, Atari treated their inner workings as trade secrets, in particular their custom graphics and sound chips. Third-party developers could only receive technical documentation on those secrets if they signed a nondisclosure agreement. (Ted Nelson complained bitterly about this situation in his introduction to The Creative Atari.)
Something changed at Atari, though, and the company shifted course. In 1981, several Atari employees began publishing articles in the computing press on the Atari's internals: player-missile graphics, display lists, sound, and more. These articles started in January 1981 (in BYTE and Compute!, which you linked to above) and continued into the next year.
In 1982, this information was collected and published in a single book, De Re Atari, which—for Atari enthusiasts and professionals—quickly developed a reputation as the Bible for the Atari. It was one of biggest sellers for the Atari Program Exchange.
As blogger Atari_Ace points out, it's not that player-missile graphics et al. were unknown by third-party developers before this information was published. No doubt some reverse-engineered the secrets on their own, or had access to the knowledge via those who worked for the right companies. But the revelations of the De Re Atari authors formally introduced the technology to the computing public.
Strangely enough, Atari later became known as one of the more open companies in the emerging microcomputer market. The Atari BASIC Source Book includes the complete source code for Atari's BASIC cartridge as well as a detailed explanation of its workings. Ian Chadwick in his Mapping the Atari thanks the company for its "'open system' policy." Make of that what you will.