Most (not all) of the seized fentanyl is not being smuggled by immigrants coming over the border. The arguments Republicans make are that the migrants are exacerbating the situation by diverting customs resources and that those numbers could be skewed because there is equipment to detect drugs at ports of entry but not between border crossings, so the seizure rates could be higher at ports of entry out of proportion to the trafficking rates.
Obviously this is politics and people can disagree with their arguments, but this is one of the other favorite "don't lie but kind of do" games. The claim that detection rates could be higher at ports of entry isn't outrageous, there is some logic to it, but since by definition we don't know what the rate of undetected trafficking is in each location, there is "no evidence" for their claim. This is not equivalent to it being proven false, but that will often be implied.
Obviously this is politics and people can disagree with their arguments, but this is one of the other favorite "don't lie but kind of do" games. The claim that detection rates could be higher at ports of entry isn't outrageous, there is some logic to it, but since by definition we don't know what the rate of undetected trafficking is in each location, there is "no evidence" for their claim. This is not equivalent to it being proven false, but that will often be implied.