Slide 11 appears to say that a large number of cases were hospitalized. If this interpretation is correct, we are talking about a diagnosis from a doctor, not self-reported data.
In this case a look through GP's recent comment history reveals many comments with disinformation and fear-mongering.
One thing that the paper is pretty clear about is that in order to proclaim a "case", the CDC reviewed medical records and interviewed the healthcare provider looking for specific criteria. It seems like all criteria include at least one test, e.g. EKG, in addition to symptoms reported by the patient. It's not like they ran a SQL query over a bunch of dudes reporting chest pain---the CDC did their homework. If anything, I have more confidence in the CDC and the vaccines now than I had before reading these documents.