It is funny to occasionally see it explained like 'on Reddit you can use ...' and think '..dude, markdown, just tell them you can use markdown' (and then realise oh right yeah ok, your way is probably clearer to them and you probably don't know it as 'markdown' either).
Reddit's Markdown flavor is a bit weird though. It got closer to CommonMark with New Reddit, but the rest of the UI got worse, and people using Old Reddit don't get the formatting the new version supports, so things like code blocks are often broken.
Original Markdown didn’t have fenced code blocks either.
Yes, Old Reddit Markdown is much closer to original Markdown than to CommonMark. New Reddit Markdown is closer to CommonMark.