So here is where I have a problem with that statement. When I am reading specifications, I generally look for overarching statements and specific statements. To me, what you quoted, was an overarching statement, while the quote I provided was a specific statement from that element. Because that statement is more specific, I give it more weight, and assume that I overrides a more overarching statement.
Given that mindset, when I read that statement and an explicit reference to a SPACE character(GitHub Flavored Markdown Spec) when it would have been as easy to reference a WHITESPACE character (GitHub Flavored Markdown Spec) or a UNICODE WHITESPACE character (GitHub Flavored Markdown Spec), makes me believe that was specifically put there for a reason.
Does that reading, and hence my confusion, make sense?