The harm is mostly in the water that must be used to mix up the formula and clean the bottles, which wasn't safe in these areas, and many of the new parents targeted by Nestlé didn't have the facilities to boil water or even know that they needed to. It didn't help that the instructions on the cans were in English. (Of course, advertisements promoting formula as the modern baby-feeding method of choice were in the countries' local languages.) The formula was also expensive, so many families overdiluted it to make it last longer. It's not like they could just switch back to breastfeeding—by then, the babies were often so used to the formula that they'd refuse a boob and/or said boobs had dried up. The result was an estimated one million dead babies every year from malnutrition or diseases contracted from dirty water or bottles.
All of this led to massive boycotts in the late '70s and early '80s, forcing Nestlé to put the fake nurses out to pasture, but not before insisting that the real problem was access to clean water, and hey, did you know they also happen to sell bottled water? (Weirdly, this didn't stop them from pushing to declare clean water a "need" rather than a "right" at the 2000 World Water Forum.)
Nestlé
Over the ensuing years, the company has continued to fight off accusations of unethical international marketing of their infant formula, including some pretty undeniable ones. In 2017, six Nestlé employees were found guilty of bribing patient information out of Chinese hospital staff for the purposes of marketing their formula and went to prison. Incidentally, when China uncovered a formula manufacturing shortcut that killed even six babies, in which Nestlé was also implicated, a farmer and a dairy manager were freaking executed. They have their problems, but they take food safety very seriously.