Heya! Handwave is fine for this, since it's common knowledge — dream thieves are humans with the ability to pull out things and people from the dreamed world and into the waking world. They can also use dream reality to change woken reality. They're particularly able to do that in areas with high density of dream magic, like the Cradle, which work as amplifiers. Unfortunately, this also heightens the volatility of the Cradle storms.

Per what other folks learned during the trip, some dream thieves like to use dream artefacts for lower-level or subtler dream changes.