When they were traveling through the breach, there were various images flashing by.... one of those images was Supergirl in flight.
Although, I guess it doesn't need to be another Earth, because it looks like images from other times flashed as well.
The images that flashed by:

Supergirl

Green Arrow w/goatee, confirmed to be appearing in Legends of Tomorrow (from the future)

Jonah Hex, confirmed to be appearing in Old West episode of Legends of Tomorrow

Legion of Super-Heroes flight ring

The 1990 Flash