Well, yeah. I'm not saying he disregarded it completely.
I'm sure he always intended for there to be more to Guts, he must have had at least the bare bones of the flashback in mind before launching into something that lengthy.
But the starting guy, and the storytelling, and even genre, were pretty different in the earliest chapters, )(specially if you throw in the prototype chapter, which doesn't officially count but does show what he wanted to do) which is only natural for a new author on a new story. I think maybe he had the idea of the backhistory, but maybe didn't ever plan for it to turn out as long or detailed as it did.
Almost certainly the very first page of the series where Guts is knowingly having sex with a demon would have been handled differently if he had a stronger grasp on the whole at that point… memorable visual though that is.
It's not a criticism. I could say some of the same things about Neil Gaiman's Sandman. The first arc of that was radically different from anything else the series would be and the lead character wasn't quite figured out yet... just how it is.