I don’t think they replaced the sun-dried raisins with honey, otherwise the honey taste would be a bit too strong, too obvious. Instead my guess is that they used it as an additive in the final product, if the resulting passum was too dry. So it’s a bit of a post-production additive.

I’m saying this because

1. As the text mentions, there’s a shortcut for making passum - to boil the must instead*. If the issue was space, time, or labour they’d likely do it instead of relying on yet another parallel culture.
2. Grape sugar content might vary quite a bit from one harvest to another. And yet it you’re doing large scale production you want some consistency.

*you can do this at home with some grape juice. Even the unfermented version is delicious, it becomes syrupy. It goes great on vanilla ice cream by the way.