I found a study from last year that says summer and winter can both cause heart attacks.

https://news.sanfordhealth.org/heart/seasonal-heart-disease-study/

Here's a quote from another article:

"During high temperatures, the heart must work harder as it tries to cool the skin. It pumps up to four times as much blood per minute as it would on a cold day," says cardiologist Kimberly Parks, MD, assistant professor of medicine at Harvard Medical School and medical director of Synergy Private Health.

So, in other words, bullshit.