That's the thing with ancient wisdom. Over thousands of years, it either stands true and turns into common sense, or is replaced with better modern wisdom.
It's also because it was meant to be read by lords and nobles to teach them the basics of war. A lot of them were EXTREMELY disconnected with reality and didn't even fathom basic shit like "Oh yeah your troops need food. They can't just forage mid war like animals would"
Reminds me of that time I drove an hour across town to one of our remote offices... Only for the printer to be unplugged AND the cable to be sitting in the hallway (you could trip over it!)