Casting a spell and holding it uses visible components the hobgoblin could react to.
I’d think it’d take at least an arcana check to know that the spell you’re holding is Jump. So yeah, he could react, but would he necessarily be able to react in time to stop you from casting the spell? Not to mention, if he reacts just barely not in time and jumps haphazardly, one could argue the Wizard jumped farther and therefore wins.
If he doesn’t know what spell you’re casting, that means that he’s even more likely to assume that you are trying to cast an harmful spell, making him attack you. And casting a spell takes an action, basically a third of your turn if you want, so the hobgoblin has at least 2 seconds to react, if not more. And thats plenty of time to stop himself from jumping.
