It was allowed because SRFI 33 was and is withdrawn, and nobody was going to put effort into a withdrawn SRFI.  SRFI 142 (also withdrawn) and SRFI 151 (current) are its successors.  At that time it was apparently normal to allow early drafts of a SRFI to be in plain text or any sort of markup, including org-mode; only the final draft absolutely had to be HTML.