The Solar System as ecosystem idea is a Sci Fi concept based on a very different time scale of life than the one we know. It doesn't require technological space travel as there are ample bodies moving through the system to spread life that evolved in deep space. The come around point of the cycle, which is to say the point at which life, deposited in a gravity well, returns to deep space doesn't even theoretically require technology as objects skim through the upper reaches of the atmosphere, where there is some life, all the time. It's a thin stretch, but we're not talking about a human scale biomass, more like a hibernation heavy bacterial colony.

This isn't to say I subscribe to the theory! I just find it a very interesting thought experiment. I think it originated back when there was some debate about whether or not all the key ingredients for life were present on the early earth.