ID also makes advance predictions about the sort of as-yet-undetected biological features might reflect intelligent creation ratheer than natural origin (e.g., complex chemical processes that don't provide any benefit if even a single component isn't present would be unlikely to develop as a gradual process). SETI has made a number of guesses based on little or no evidence (e.g., the claim that the probability of intelligent life naturally evolving is high enough, and the number of suitable planets is large enough, that intelligent life must be out there somewhere).
The difference is equivalent to that between painting a target on a wall and shooting a bullet through the bulls-eye and shooting a bullet through a wall and painting a bulls-eye around the hole.
In both cases, we have people trying to paint a target around a wall they can't see or prove exists. In the case of ID, they are trying to define a target called "evidence of non-natural design in life" and in the case of SETI, they are trying to define a target called "evidence of extra-terrestrial intelligence". Neither really knows what for sure what the evidence really looks like because neither of them have any hard evidence, so they are left to guess at what the evidence would look like. Both, ultimately, are looking for evidence of intelligence. As such, both have come to the same conclusion. The best way to look for evidence of intelligence is to look for things that can't be explained by a natural process alone. And in both cases, even if they were to find such evidence, a skeptic could claim that their supposed evidence of intelligence is simply evidence of some unexplained natural phenomena.
Even if the SETI people found a narrow-band signal of the kind described, would it prove extra-terrestrial intelligence to a skeptic? I doubt it. They could simply argue that it was produced by some yet-unexplained natural process. And would they be wrong to do so?
This turns out not to be the case.