what lore is official then, we need to work with something right? Your argument can be said about absolutely everything, i believe all writen lore is official, weather it is true or not is another case
To me..
Official Lore is the Lore that is True. It happened. It is like that, during the time periods mentioned. Etc.
Possibility Lore is what isn't truly know and could be possibility of being true or being something else.
False Lore is truly incorrect and are just lies.
Official Lore is the Lore we experienced in the games or are said by Bethesda Softworks themselves. There are many factors that plays a role here though, such as time and game mechs.
Possibility Lore is what is mentioned in the books of Tamriel as well as being statues, and many other things. This lore could be possible but we truly do not know.
False Lore is truly incorrect and can easily be proven to be incorrect...
Now.. The In-Game book mentioned earlier counts as Possibility Lore since I do NOT remember Bethesda saying that it is correct. However, Bethesda did mentioned that little story about Talos which you've said so that is Official Lore.