I think there's some confusion in your comments. The HTML comments aren't a templating language, there's no (or little) post processing when displaying this content.
They're annotations that indicate where a 'block' of content starts and ends.
The JSON data stores some values set by users for a block that can't easily be parsed from the HTML content.
The content is stored as HTML as most content a user creates is static HTML, so when displaying content much of the HTML is displayed verbatim (but without the HTML comments), and then there's only a little bit of progressive enhancement for dynamic content.
I think you're also conflating some terms, like 'themes' and 'blocks', these are two different things and launched years apart.
They're annotations that indicate where a 'block' of content starts and ends.
The JSON data stores some values set by users for a block that can't easily be parsed from the HTML content.
The content is stored as HTML as most content a user creates is static HTML, so when displaying content much of the HTML is displayed verbatim (but without the HTML comments), and then there's only a little bit of progressive enhancement for dynamic content.
I think you're also conflating some terms, like 'themes' and 'blocks', these are two different things and launched years apart.