As one noted in the article ""almost certainly an underestimate", because a number of climate-health relationships are not measurable due to lack of adequate data".

Milder winters expand the range of disease-carrying insects, increase risks of wars or famine with less water, mass migrations leading to political instability, more extreme weather events like draughts and flooding leading to failed crops and other natural catastrophes...

Meanwhile much of the WHO estimate is for people dying due to heat exposure. Easy to quantify, important to know - but nowhere near the full picture.