What's more is an expression that's used when you want to emphasize that the next action or fact is more or as important as the one mentioned. War doesn't bring peace; what's more, it brings more chaos. Or your example.

How to use "what is more"? - English Language Learners Stack Exchange

What's the difference between these types of adjective usages? For example: This is more of a prerequisite than a necessary quality. This is more a prerequisite than a necessary quality. (without ...

more of a ... vs more a - English Language Learners Stack Exchange

The modifies the adverb more and they together form an adverbial modifier that modifies the verb doubt. According to Wiktionary, the etymology is as follows: From Middle English, from Old English þȳ (“by that, after that, whereby”), originally the instrumental case of the demonstratives sē (masculine) and þæt (neuter).

The stories may be make-believe, but ALSO much more than make-believe (that in the sentence): It will among other teach them the morals of the Agta, the myths and how they see the world around them. Possibly even prepare them for other skills - how to spot certain foods, teach them more words in their language etc.