Just a quick note:
Water and fire means Kan and Li. Kan has two yin lines on the outsides and a yang line inside, so it means even though water is soft, it is internally hard.
Li has two solid lines on the outsides and a broken line inside, which means even though fire is strong, it can be controlled. The relationship between Kan and Li and external and internal martial arts is very deep, so what he said there has more than one level of context.
In regard to the whole body not using strength, he specifically said 全不用力 completely don't use "Li" Li refers in this case to the strength of individual groups of muscles such as what you would use when you are working. It doesn't mean the body doesn't have strength, it means the strength is not obtained by separating the body into individual movements like you would when you swing a hammer or write with a pencil or snap your hips to throw a punch. Instead, you should use the strength of the whole body which is controlled by the mind, he says 專一意志。志者心也 "use one intention and resolve. resolve refers to the resolve of the heart." This means you need to use xin and yi together to practice power correctly. The meaning actually is considerably deeper than that if you look at it from the perspective of five element theory, but I won't get into that here.
I don't think anyone ever intended for Taiji to be practised without power, but certainly the way that taiji gets its power is considerably different than other martial arts. From what i saw of the article, he is constantly reinforcing that yin and yang need to be balanced in order to get the key practice of taiji.