The study includes vaccinated and unvaccinated individuals and documents respective risks. It spells this out explicitly in the Results paragraph: "but was low

The study includes vaccinated and unvaccinated individuals and documents respective risks. It spells this out explicitly in the Results paragraph: "but was lower than the risks after a positive SARS-CoV-2 test before or after vaccination (11.14 [95% CI, 8.64–14.36] and 5.97 [95% CI, 4.54–7.87], respectively)." And more extensively in summary: "First, the risk of myocarditis is substantially higher after SARS-CoV-2 infection in unvaccinated individuals than the increase in risk observed after a first dose of ChAdOx1nCoV-19 vaccine, and a first, second, or booster dose of BNT162b2 vaccine. Second, although the risk of myocarditis with SARS-CoV-2 infection remains after vaccination, it was substantially reduced, suggesting vaccination provides some protection from the cardiovascular consequences of SARS-CoV-2. Third, in contrast with other vaccines, the risk of myocarditis observed 1 to 28 days after a second dose of mRNA-1273 vaccine was higher and similar to the risk after infection. Last, vaccine-associated myocarditis was largely restricted to men younger than 40 years with 1 exception; both younger men and women were at increased risk of myocarditis after a second dose of mRNA-1273." It does so by including "5,934,153 had SARS-CoV-2 infection before or after vaccination."