Imagine a story of a b c, all of them were beloved by the reader.
Halfway through the story, c disappear without any notice, reader were pissed off and they tried to explain why based on the clue presented in story.
But after all that think, all they could make is a bunch of theories with flaws and holes, in desperation, they contacted the writer for the truth.
c was killed, the writer said, reader was shocked and angry but they cannot do anything, because they're not the writer and this story wasn't writen by the them.
That is literally what happened in this chapter, with Battler becoming the writer of the story, and with that he can easily confirm the corpse of Kinzo.
But Gold truth isn't perfect, it's on par with red truth as someone mentioned in the manga, like he couldn't just claim "George is alive" when it's clearly showed before that George died.