Most people don't think to press up and down to control something in game because you're four zones into the game - two games and four zones into the series - and you haven't once been conditioned to use up and down on the d-pad to interact with the world. You've pretty much just done three things throughout the whole series: run, jump, or spindash.
Then you get to this barrel, and it suddenly asks you to use up - which can make Sonic look up, but in practice nobody ever actually uses - and down - which you only use if you want to start going fast or turn your already fast speed into an attack, so isn't really the first thing you think of when you have a wall in front of you. So the player tries the primary tool in his arsenal: jumping. And he sees how it has a clear effect on the barrel, and in fact comes so close to working, it's easy for him to figure that he's just not quite doing it right and keep on trying.
"HURR BUT I GOT IT ON MY FIRST TRY, IT'S SO EASY, HOW DID PEOPLE GET STUCK LOL"
Well, you know, good for you. Your brain managed to match up with the designer's brain at that moment. You got lucky, the same way I'm sure someone out there figured out the goddamn mouse in King's Quest 2 on their first playthrough. That doesn't change the fact that it's an extremely unintuitive element that many people got stuck on.
If it was just one person complaining about something, then yeah, maybe it's that one player's fault. If it's two, three, four, five people, well you're starting to get a potentially legitimate issue. When it's the majority of the people who played the game and got that far, then it's pretty fucking clearly the game's fault.
