Regardless of the minutia of the numbers, the recommendation to get vaccinated regardless of whether you have had COVID in the past or not is based on the fact that a very high number of people in the US and worldwide "think" they probably had covid but aren't entirely sure because they never got tested. The vaccine for everyone is easier and cheaper from a public health standpoint than to have people taking serology tests that may or may not be accurate and then waiting for the results in order to find out if you should get the vaccine or not. When the fact is, even if you were previously infected, you will be even better off getting vaccinated.