If you consider wikitext to be a markup/formatting/display language, then you would expect there to be little or no gap between "one" and "two", a much bigger gap between "two and "three", and twice as big again between "three" and "four".
I disagree with that assertion. HTML is a markup language by any definition I know and it collapses all whitespace to a single space.
I would say that wikitext *is* a shorthand for HTML on the grounds that it is defined only by how it is rendered as HTML (ie. by what parser.php does). Two carriage returns isn't defined as being the start of a new paragraph, it's defined as being the start of a new <p> block. That would change if we put together a proper grammar, etc.