A thorough scrubbing is a must before painting any exterior surface. It removes the dirt and broken-down paint residues that keep fresh coats from adhering and gets rid of mildew that grows on paint in all but the most arid climates. Most contractors clean with pressure washers, but in the hands of someone unfamiliar with the equipment, these can gouge wood, shatter glass, and drive water behind siding and trim. Using a hose, a pump sprayer, and a scrub brush is slower but safer, and just as effective.
If there is any peeling — and there usually is somewhere — it's essential to scrape off any loose paint. This doesn't just mean the parts that are already separated from the wood and ready to fall off on their own. Each spot where paint has peeled loose needs to be scraped thoroughly until you can no longer get the sharp corner of a putty knife under any of the surrounding edges of paint. Then, the bare spot needs to be sanded until the paint edges are smooth.
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.