the author states "Natural immunity was estimated to be about 13 times stronger than having two doses of the Pfizer-BioNTech vaccine."

The choice of the word ESTIMATED is rather poor as it suggests rather poor reliability.

The researchers used well-established statistical methods and CALCULATED the difference in immunity and found natural immunity to be 13.06 times more effective and the result had a CI of 95% indicating a very high degree of probability that the statement is true.