In the tweet HTF does say they're facing IOA, so no way does the engine being visible at IOA but not at 9 & 3/4 make sense. You guys are right about it being the opposite, I think he just misspoke.
He even says you only see the engine while exiting 'IOA', which is only logical if he meant exiting Hogsmeade, not IOA. Therefore, the engine must NOT be visible at the Hogsmeade station, but visible within the London platform. Maybe he was just tired or something.