He was curious whether or not this was an error in the original text, or a translation error. So I took a quick look:
“The three countries of Ylisse, Regna Ferox, and Plegia make up the continent of Valm, known as Archanea in previous Fire Emblem games and Valentia in Fire Emblem Gaiden.”
FE fans can already spot the multitude of errors above with simple searching.
The original Japanese text (with my translation):
Also, the three countries of Ylisse, Regna Ferox, and Plegia make up the continent that was known as “Archanea” in previous series entries. The continent of Valm meanwhile resembles the continent of “Valentia” which appeared in Fire Emblem: Gaiden.*
*-Used localize names to keep it simple.
This seems more in line with the lore we know of too. It seems it was just a translation error and not one that was in the original text of the book.
What likely happened was they read the descriptor above as talking about the same continent, rather than the small comma separating them talking about two different continents. Quite literally, it reads like:
Also, Ylisse, Regna Ferox, and Plegia are three countires that exist on the continent that was in previous games known as Archanea, Valm appeared to resemble Valentia from Fire Emblem Gaiden.
Seeing the above makes it a bit easier to see how it happened (talking about previous games, then the mention of Gaiden). The translator perhaps did not have key context clues from the games either, having never played them/being unfamiliar with them.
That is all speculation however. Lastly is a summary info-graphic for use if needed to share this post in simplicity.
If you spot any more potential errors, please let me know and I will look into them!