The action=parse module of the API seems to ignore the text parameter when the content-model is set to proofread-index or the title is set to any page in the Index: namespace.
Ex: APISandbox link.
Instead of parsing the wikitext supplied, it outputs a unfilled version of the template at MediaWiki:Proofreadpage_index_template.
Okay, I was looking in the wrong place, looks like the ProofreadPage extension's function unserializeContentInWikitext() actually does some pretty weird stuff. Looking at the code, it seems to search for any templates and then rips out the parameters of the templates to add them to an array which is then passed as parameters during the creation of an instance of the IndexContent class. So anything not passed as parameters to a template (any template) is completely ignored. Also, passing {{ bold | Pages = [[link]] }} will render as the html equivalent of {{:MediaWiki:Proofread index template | Pages= [[link]] }} which is not the expected output.