This claim is actually backed up by the CDC, and the British Heart Foundation itself is a very reputable research charity. The article uses numerous credible sources and provides detailed explanations of myocarditis, pericarditis, and how vaccination affects heart health, enhancing its overall credibility.
It's fair to say that this claim is true, though all sources emphasize that the risk is quite rare, and significantly more rare than the increased risk of heart inflammation from Covid itself.