What I didn’t remember though and only looked up
recently: the MSRP was $60-70
Notice that Super Mario World isn't even in that ad. That's because Super Mario World was a free pack-in game for most of the SNES's lifespan, at least in the US. So the MSRP was effectively $0 for a lot of us.
is a 5 hours long game,
That means you spent about 10-15 minutes on each level if you did a minimal playthrough and skipped all secrets. A brisk clip for an initial playthrough but it seems reasonable for an experienced gamer I guess.
There's no right or wrong way to enjoy a game, so if you are the type to power through a minimal playthrough and never touch it again... cool. No hate. That's your style. I have definitely done that with some games.
On the other hand if I enjoyed a game back then I definitely replayed it multiple times, so "5 hours to completion" definitely did not equal "5 hours of fun" for me and a lot of other people.
9 [hours] if you are going to 100%
Now this number is objectively junk. To get all 96 exits you're going to have to beat all 73 levels and you're going to have to beat some of them twice so you will play a total of 96 levels.
Some of the hidden worlds and hidden exits are tough to find. Doing it in 9 hours on your initial playthrough means you're hitting a new exit every 5.6 minutes. Not realistic.
Obviously it can be done in 9 hours or even much less with savestates, walkthroughs, playthrough vids, whatever, but that doesn't mean the game only has 9 hours of fun.