I don't think the phrase you quote is intended as a dismissal of the data. It comes across as an acknowledgement of the data's limitations, and a caution about what kind of conclusions can be drawn. During my statistics course in college, one of the first sections of the course covered sampling, and the various ways sampling can be done badly, thereby biasing the sample, and thereby leading to conclusions about the sampled population that don't reflect the reality of that population.

Cautioning against that failure mode strikes me as intellectually honest, not anti-science.

how is he dismissing data? questionable data should be treated as such... why would a linear extrapolation be a good fit here?

I think they are implying that blood sampling is not a random sample of people (more likely to be from someone who is sick), not that it is inaccurate.