The only time it's acceptable to mix water in the paint is when you're using a deep or ultra deep base paint to reduce its stickiness, which is rare with new paint technology. Dark primary colors are composed almost entirely of tint that makes it very hard to work with without adding water. (You never use a sprayer and need to thin paint.?) If i was still in the industry I'd take the time to make a better article than this. Take this for a grain of slat. Use a reputable painter or someone you know you can trust or has been referred to you by someone you trust. I wouldn't hire anyone I had to watch like a hawk to make sure they're not screwing me.
Back to the article. You can add water to all latex based paints / thinner to oil based paint. The tinting base has absolutely nothing to do with it. Say you are working outside and throughout the day you have to add a little water to keep the same consistency. If somebody really tried to add 20% to 50% water they no longer would be painting they'd be performing a whitewash or pickle finish.