It was the vaguest it could have possibly been tho.
Litten yelling at a stoutland is a pretty far cry from Stoutland actually dying. Maybe if this was a Bonnie episode, but as it is, I dont see it.
It's not the yelling that was bad but the fact that Stoutland wasn't waking up. You could interpret it as 'oh well, Stoutland is just in deep sleep' but the fact Litten had a nightmare implies the worst case scenario: Stoutland falling asleep and never waking up. So, Litten basically had a nightmare where Stoutland dies.
All of this is further emphasized by Rotom's comment on how Stoutland looks old and Stoutland's haggard appearance.