it's not written up explicitly here, so if you're wondering about the rating Musashi gives at the end, he's referencing the battle of seikigahara, which was the decisive conflict which solidified the tokugawa shogunate's control over all of japan; the final conflict before the Edo period.
over 160,000 men fought in the battle, in a bizarre series of clashes between samurai, cavalry, riflemen (yes, rifles! from the dutch), and siege weapons. I don't actually know too much about the actual logistics of it, but it was pretty insane.
Although the real life Musashi never formally confirms he was in the battle in his own writings, several extant sources say he was there. He was 16 at the time, but already an accomplished swordsman. One extant account claims "Musashi's achievements stood out from the crowd, and were known by the soldiers in all camps." We do know Musashi participated in six battles during his lifetime, and it's highly likely this was one of them.
I'd say that, from him, this constitutes pretty high praise for retsu.