Furthermore the article clearly states:

Testing on frozen shrimp from the distributor, Indonesia's BMS Foods, also tested positive, the FDA said.

The shrimp was in fact contaminated. But as a precaution; with anything Radioactive, it is always presumed that anything that is in contact with a contaminated object, is also in and of itself inherently contaminated.

I recommend you watch some Kyle Hill on Youtube; he explains clearly how these things work. Radioactive contamination is no joke. It is handled with this level of caution for a reason. It kills people quickly if left unaddressed.