Seems more like Garnet foresaw an scenario which both died training with Pearl, rather than a future spoiler.

Especially since she says “The children are playing swords”, meaning this wouldn’t be possible in a long term future, and CN would never accept to transmit a child’s death.

I'm still like 89% sure this Steven will meet Rose like this. Then there will be a choice as to who will reform.

"Me… or you?"