those who were vaccinated and reported a cardiovascular effect?

That's not how this study was comprised. They chose children that were double vaccinated, not patients who where showing symptoms, that happen to be double vaccinated.

OP thinks it means that 30% of all children who were vaccinated developed a "heart problem" so yeah, it very much matters who the study is about.

That is correct.

Do you think that the study concludes that 30% of all vaccinated children develop a "heart problem"?

Again, yes. In addition, the paper does have a conclusive statement at the end. Do you need me to read it to you? You fucking infantile vaxxtard.