Actually some anime materials like bios and radio shows do count on one of those materials it revealed Chibiusa's manga age of 900 years old. While somethings have questionable canonicity in the extra anime materials the Shitennou's backstory is confirmed.
Even if it counts, if it's not shown in the anime itself then it doesn't count for me, because audiences should be able to know about the characters and their backstory, what they do etc from the storytelling in the anime itself, without having to rely on other sources or materials, otherwise it's just bad storytelling.
Take the Shitennou's backstory for example, in the 90's anime there was hardly anything to indicate or even imply about their past lives, there was nothing about who they were in their past lives, where did they came from, how did they come to work for Beryl etc, there was NOTHING. If I hadn't read the manga, then I wouldn't have known about the Shitennou's backstory or where they came from etc.
But at least in Crystal, there were flashbacks about their past lives, and detailed explanations about who they were and what they did before they were brainwashed by Beryl, and THAT is backstory.
But in the 90's anime there was nothing about the Shitennou's backstory or past lives, they were just portrayed as merely Beryl's disposable pawns, and nothing more. And knowing their backstory from the manga, I was so disappointed that they did not explore the Shitennou's past and background in the 90's anime, they were only just known as Beryl's minions. If one had not read the manga, or watched Crystal or PGSM, then he/she probably would not have known about the Shitennou's past and their backstory. Like I said before, I just cannot connect with the Shitennou in the 90s anime in the same way that I connect with the Shitennou in the manga, Crystal and PGSM. I still like them in the 90's anime though, but I much prefer them in the manga, PGSM and Crystal.