It is odd that more people didn't have symptoms, but given that the symptoms preceded discovering that her apartment was next to the toxic waste it's more likely than not that this is legitimate.
Having lived in a new apartment building, the smell was very strong even without a superfund underneath. What are the odds every new apartment tenant is being poisoned, but most don't have asthma or other sensitivities to make it obvious and are training in a profession that teaches them to look for it?
It's possible that every tenant is being poisoned (and I'd suggest that this is not good either). There's some evidence that the new car smell isn't good, and the articles reference that problem being with VOCs (link to study https://www.sciencedirect.com/science/article/pii/S016041202...).
I don't think people are doubting that she has symptoms, just that they may be exacerbated by anxiety. Anxiety that she has learned she lives near toxic waste and has a gadget she bought on the internet that shows big numbers.
“This apartment complex had no physical or written appearance of danger from chemicals, other than a brief and vague note about the presence of “agricultural chemicals” buried towards the end of the “Resident Handbook.”