I am quite surprised

Suprise means your prior are likely to be wrong. Congratulation on having found an opportunity to update them in the right direction.

Talking about environment I suppose first we should make it more eco-friendly, finally get rid of fossil fuels and stop transforming the planet we live on into a heap of rubbish.

That's not something you can buy with a few million. Effective Altruism is about focusing your efforts where you can actually get a good return for the money that's spend. It's not about listing problems and tackling the most "pressing" problem regardless the return that you get for your money.

In this case getting rid of mosquitoes via releasing large numbers of sterilized male mosquitoes might be done money that's otherwise payed for anti-mosquito spray by individual people.