(Less so if others are exculpatory. Which is not the case here. A known false positive is not conclusive proof, but neither does it explicitly clear a source.)
They already told you clearly, so it sounds like you're trying to browbeat them with a fallacy of equivocation over the scope of "it".
These things are not equivalent:
1. A biological test for a parasite in a sample of equipment/material from the factory. (Claimed as false-positive.)
2. The analysis of the supply-chain for the food that sickened people which led to that facility as a common factor. (Presumably still accurate.)
___________
To use a criminal-investigation metaphor: Suppose a dead woman is found covered in knife wounds, and the circumstantial evidence leads to one Knifey McStabberson, since he (A) previously dated the woman and (B) sent a death-threat once via text-message, and (C) has a collection of hundreds of suitably-sized knives. Investigators notice one that looks bloody, and test it... It's her blood! Wait, oops, no, false-positive, it was something else.
What is your takeaway from the story so far? Is it that Knifey McStabberson himself is "a false-positive" and obviously totally innocent... or is it that it would be a good idea to check more of the knives scattered around his house and in his car?
Meh, I don't see how I could make anything worse, with how they were featured in Leave it to Cleaver, and recently dissed in that song by the Full-Tang Clan.
You don't always get a positive test, but this doesn't necessarily prevent you from concluding the cause of food contamination.