That's the bit I'm not sure about. old Seth says that any super-advanced civilisation will be indistinguishable from machines, or will be biological/machine hybrids, or something like that, doesn't he, so how is he so sure that SETI will be able to detect them, since it only looks for radio communications? Why is he so sure that a biological/machine hybrid civilisation will use radio communications, when surely they'd have long moved on to something much more sophisticated?
We can't be sure at all, it is just the best best we currently have. However, given that almost anything we do emit electro-magnetic radiation, in my honest opinion it is a good bet the same is the case with ET. It may not be powerful enough to pick up - but then again, it may.