So I decided to look up the actual trope entry of
Writing by the seat of your pants and turns out there is an entry for Berserk there.
And shit, you guys thought that the lore was in trouble?
My guess is that Miura has the ending confrontation with Guts and Griffith worked out, but beyond that had nothing. It also explains some of the out of character moments Guts had in the early volumes.
Which, to all the people who are now kinda panicking, is fine. Planning is immaterial to some writers. Breaking Bad was written in a similar fashion, and it's continuity is one of the strongest in TV.
So yeah, I stand by my thoughts that Miura probably had no idea about the greater cosmology of his fiction. And that's just fine. Contradictions in continuity are the problem, not mere uncertainty of them.