The Book of Army Management says:
It is perhaps significant that none of the earlier commentators give us any information about this work. Mei Yao-Ch`en calls it "an ancient military classic," and Wang Hsi, "an old book on war." Considering the enormous amount of fighting that had gone on for centuries before Sun Tzu's time between the various kingdoms and principalities of China, it is not in itself improbable that a collection of military maxims should have been made and written down at some earlier period.
Implied, though not actually in the Chinese.