Certainly Bill Gates and his endeavors have created immense value for many.
None of these points are being contested or purported in the "line of thinking" you mention (the letter I linked I assume?)
These points are being purported/contested here:
"There is nothing worth celebrating about a world where inequality is so extreme that 58% of people are in poverty, while a few dozen billionaires have more than all of their wealth combined"
“There is nothing worth celebrating about a world where 58% of people are in poverty”.
The “inequality” and the presence of billionaires is a distraction from the real problem: poverty.