Really, saying the Kryptonians having had other colonies leaves too many "Yeah, but...." questions in place while also mudding the "Last Son of Krypton" thing. I more like the idea that Kryptonians couldn't
leave Krypton either due too high of gravity for meaningful space travel to be possible (Kal's ship being the exception as it was launched as Krypton imploded thus didn't have to fight gravity as much) or it was a banned practice as Kryptonians knew how different types of solar radiation effected them and the implications that could have on other worlds.
Here it's like, "Yeah, we did it. But gave up. Everyone else out there? Who knows what happened to them? NBD."