It is not what I expected, because I just never considered the possibility of the young Musashi seeing future events (flash-forwards? as flashbacks are for past events). I was thinking more of alternate reality, where (most of) the original events take place, with the young Musashi participating in place of the regular Musashi. He actually does that, standing in for the regular Musashi to some extent, but not as seamlessly as I supposed. In the end, he remarks something to the effect of what he witnesses is probably all in a dream.
(Ha, I thought they cut out the regular Musashi completely, but they left him in, because it would be awkward to not show him at all, only just not with his face clearly seen.)