Perhaps you should actually go to Sheldon's website and read it for yourself, rather than criticise him based on second-hand information. The late Sheldon Brown was a knowledgable bike mechanic and rider who freely offered advice to anyone who asked, and was great at explaining why rather than expecting people to take it on faith.
In America, Sheldon is a household name, though few have heard of him in the UK.
I used to do cowboy starts, most of the time it worked, except when I crashed! Such as when I forgot I put my shopping in the rear basket and my foot got caught in it. And when I attempted a cowboy going up a steep hill, stalled just before I got my leg over, and CRASH! Then there was the time forgot I had the wrong trousers on, a bit too tight to get the leg over, CRASH!
I can't remember what Sheldon's main objection to cowboy starts is, to me the obvious issue (apart from an increased risk of crashing) is that bike wheels are only strong vertically, and are weak against the lateral forces exerted doing cowboy starts or standing up on the pedals (something else I remember Sheldon cautioning about).
And no, I don't pump my tyres to the max, I can't be bothered to explain why, perhaps Sheldon's website can shed light on why I would do such a preposterous thing.