It seems like this would be extremely difficult to accurately measure for many reasons. Two big ones immediately come to mind.
1) Confounding variables. She essentially states that she looked up resistance movements and then quantified what percent of people participated and the outcome. But there are going to be tremendous confounding factors here. While she does not offer much in the way of examples she did reference the most iconic examples - Gandhi and the US Civil Rights movement. The Civil Rights Movement has some great examples, but I think India is perhaps even more striking so let's just consider that.
The British Empire was collapsing into bankruptcy in 1946 following World War 2. They were only saved by an emergency loan from the United States. At the same time this was happening the world had become divided between two superpowers - the USSR and the USA. And neither of them looked fondly upon British imperialism. India would be granted independence in a matter of months - in 1947. And over the next 18 years so would nearly every former colony of India, regardless of the level of resistance. And indeed, our author here almost certainly also listed all of these incidents as successes of nonviolent resistance from tiny minorities, even though their independence was driven largely by outside factors.
2) The meaning of protest has changed in the age of the internet. In the 1960s organizing was extremely difficult and travel was expensive. Seriously, imagine trying to organize things when the best you have is a phone where you have to manually dial each and every number (and long distance was pricey!) or things like hanging up signs on street lights. And people getting places was costly. A one-way domestic flight could set you back nearly a thousand dollars. The net effect of this is that in times past a single person represented far more people than just themselves.
However today this has radically changed. Organizing people is completely trivial and free. Travel is still a burden but it's become much cheaper and easier than ever before. This also means that protests mean something entirely different. if there are a million people protesting something, it's entirely possible that these million people don't necessarily represent all that many people other than themselves.
This also makes me wonder about something else, but since it's a distinct point - I'll put it in another post.