The Turtles assumed Shredder and Krang are trapped in Dimension X for good when they destroyed the Technodrome, but if the Technodrome's portal still works, Krang and Shredder should be able to return to Earth.
When April sees Krang's android body, she asks aloud what it is. She has seen it numerous times over the course of the series, so she should already be familiar with it.