We use the book’s red keep because we explicitly are basing the entire project based on the books not the hbo series.
We can take aspects from the hbo series but the hbo red keep is pretty vague in layout, pretty ridiculous in some aspects (like the skyscraper tower connected by 20 stone bridges to Maegors), and has a tendency to change layout a lot.
The book red keep on the other hand has pages upon pages of lore which Iwan, WRMD and dozens of others took ages to comb through and make work, its part of the reason why im against a full Red Keep redo rather than a gradient update.