Curious. I certainly must have missed the 500m Indians who did when I visited.

Although if you follow the claim to the linked source, it's a report by an NGO which I've never personally heard of (WaterAid), despite the fact that I'm heavily involved in aid work in Southern Africa, and it's also unsourced. This makes me slightly skeptical, but what's a little bit more fishy is that they didn't do any fieldwork to get to the result, and claimed that they calculated it from UN and WHO numbers. Going to those very UN and WHO numbers cited in that report, which I would trust, they define having a toilet as a proper indoor flushing toilet versus just having a standard pit latrine, which then makes that 500m number makes sense, because the vast majority of them would have used pit latrines back then (and indeed, it's a common thing to see, especially in rural areas).

Practically speaking, it's not open defecation in the way it happens in slums, as a pit latrine is just a bush toilet that you eventually pay (or force, as it is in some African countries) someone to empty and take to a disposal centre. Indeed, when I was a little boy, I helped my grandfather to build some on the farm that he owned for the farmworkers that stayed on the property. They're not that hard to build yourself, and in then rural Rhodesia (now modern day Zimbabwe) it was going to be prohibitively expensive to build flush toilets for all of them. Even on some of the farms of my extended family, bush toilets were the only option available, and bear in mind this was with a well off white minority. I'm sure they're still probably used today by the locals, although the rest of the farms have gone to waste since they were expropriated by Mugabe... but to come back to your point, I don't think I can assign epistemic credence to your claim that it's an 'established fact', given this information as well as my personal experience.

As a result, it makes sense that I didn't see any open defecation despite those numbers, because people who used pit latrines would be included in those very numbers. It's weird that I've never heard of this problem in India before though; it's quite an interesting rabbit hole because it seems there's a fair effort to try fix the problem, lots of good studies and data on it, and there's a lot of international media coverage on it unlike African countries, which would be far more receptive to it as many are doing very little about sanitation right now. I wouldn't say something like this is well known outside of the international aid space though, even though it's just the presence of basic sanitation and running water. People are surprisingly ignorant about developing countries, and organisations in the west can sometimes struggle to actually comprehend the situation on the ground.

There's a government program mentioned in the article that seems to be doing well by the looks of it due to newer numbers on Wikipedia, and according to a World Bank survey mentioned on the page it seems that most people are using them, which is good because at least there's uptake. Indeed, any industrialising country has to go through this stage: in Victorian London, there was obviously the Great Stink and infamously throwing fecal waste out of windows onto the street, and it was only with Bazalgette and building the outfall sewers that London's cholera outbreaks came to an end right at the dawn of the 20th century (Ackroyd's biography of London has a good section on this). It would be a great template for African countries to borrow and mimic, and it looks great costing wise too. Definitely a no-brainer thing, but getting the Zimbabwean (and indeed any sub Saharan) government to do anything remotely useful for its people is like herding elephants... never going to happen.