Yes, but this is only the case because we deal with an agent with known attributes. Especially in this case where the agent is human we have quite a good grasp of his methods, motivations, abilities and, very important, his limitations.
This is obviously not the case with an unspecified agent who supposedly did X with unknown methods and for inscrutable purposes. Such a designer is compatible with just about any observation because you don't have a 'bounded' model of the designer as is the case with human designers for instance.
I don't know what, other than a metaphysical bias that restricts a priori possible causes to only mechanistic ones as opposed to the actions of agency, that disqualifies this type of scientific reasoning from the study of origins.
No, the reason why we exclude ID if nothing is known about the supposed designer is the fact that it is not falsifiable.
The problem with this statement is that you're a priori assuming that we could understand neither the methods nor the motives of the "unspecified agent." As a result, you reach the untenable conclusion that it is impossible to bound our observations of the putative agent's handiwork.
In reality it is not "obvious" at all. The physical nature of the systems in question tends to bound the solution space to a large extent. Further, human experience with those systems provides a means of understanding the limits of the solution space, as well as a ready-made set of techniques that can potentially be tested for.
No, the reason why we exclude ID if nothing is known about the supposed designer is the fact that it is not falsifiable.
And thus neither is its alternative.
I would say that falsifiablity in the sense of positive verification by direct observation cannot be had in any historical inquiry. Requiring that the postulated entities necessary to origins theories have to be directly observable if they are to be considered testable and falsifiable and therefore scientific would rule out out common descent as well as ID. There are myriad Darwinian hypothetical postulations of past, unobserved and unobservable 'events' that purport to account for present biological facts and data that cannot be directly tested either.
Cordially,