This is what I mean: Does Science Work by Falsifiability?
There is a distinction between saying:
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The hypothesis is not falsifiable so it is not scientific.
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The scientist is diligently seeking to falsify the hypothesis.
#1 is not a good description of how science works, and that is what Popper meant by falsifiability. In contrast, #2 could be a correct (partial) description of good science. As for negative controls, ID could choose to use negative controls if they wanted, e.g.
The fact that they don’t usually specify negative controls (in biology) isn’t intrinsic to their hypothesis, and just speaks to a deficiency in their methodology.