Aaa, it’s a halachik principle which permits activities that might be scientifically seen as dangerous but are accepted practice in society. The gemara is saying that danger will not befall people in such a situation, which explains why yidden in Europe weren’t dropping dead all the time from lung cancer, despite having smoked day in and day out.
Smoking is not considered acceptable anymore, so its dangers have more shlita in the world.
You’re reading it literally – it’s not saying that one who relies on shomer pesayim Hashem is himself a fool, rather the pasuk means that Hashem protects people from things that should be harmful if they would be communally aware of it, just one of Hashem’s wonders and kindnesses. Talmidei chachamim who were aware of the sakanah of tashmish with meuberes nevertheless did so, because of that shmirah. They weren’t “fools.”