Apologies for the delayed response.
Updating a page template with this endpoint will not affect live pages with older content — only new pages created from the updated template.
To update live content, you would have to use this CMS Page Publishing API endpoint.
My testing suggests that HTML hard-coded into the template cannot be retrieved with this endpoint, but content in HubL modules is accessible. For example:
value:"This is the first section title. Does it update?"},
label:"Enter text for this section title",