cover is not a template/style function, and by the documentation it’s not a package for use in a document that’s more than just a cover.
It looks like to me that:
#show: cover(...)
This just means and has the effect of: replace the rest of the document from here with the result of the function call cover(...).
Since cover is not a template function (doesn’t use the document as input to the function), it seems like it should be called like #cover() and not with show? I haven’t used this package though.
See also template documentation: Making a Template – Typst Documentation An actual template function has a document parameter, called doc in the example where the template/style function is called conf. Sometimes that parameter is called body or it too, it’s just a choice.