I'd say that they're not opposing Steven as a character. Steven is having a meltdown and they're trying to calm him/restrain him. However, they don't disagree with who he is, his agenda or his goals as a character. They just disagree with his behaviour in that one moment.
Contrast that to Kevin, Eyeball or Aquamarine: they fundamentally oppose Steven's character and only tolerate him under very specific conditions. Kevin is selfish and arrogant and thinks Steven is stupid/naïve for not being like him. Aquaball/Bluebird hate how he wrecked the gem empire. They would be antagonists, because they're against Steven's big-picture goals and machinations.
That said I'm making disagreement where discord isn't needed - I'm sure whatever definition is chosen will be wonderful 😎