alt Hacker News#1 recommendation from the same link:
Keeping out of the heat: Avoid going out and undertaking strenuous activities when the sun’s at its hottest. Stay in the shade and do not leave children or animals in parked vehicles. If necessary and possible, spend two to three hours in a cool place, such as a supermarket or cinema
The tell here is that it doesn't just say 'stay at home'.
Most upper latitude European homes have been designed for mid-century cool(er) climates. They are extremely efficient at trapping and holding on to heat. Go look at any sub-reddit for central European cities over the past few days and you'll find multiple reports of 35+C conditions inside homes.
Furthermore, you can find plenty of examples over the past week in those same cities of supermarket and cinema ACs in total failure modes. Not to mention most supermarkets are closed on Sundays.
One real problem outside of the home is that there have been plenty of reported cases of drowning. Unclear if this is just statistical with the mobs of people (some of whom cannot swim, or swim well) who are forced to leave their homes to find relief.