That is because Danny listens to the products. He also understands and explains what the measurements mean and how it affects the sound. At least on the videos that I watched. And his upgrade kits make sense. Plenty of DIY'ers do the same things.
Once more, he does NOT listen to any speaker he upgrades. Ever! He says this in videos from time to time. Even if he does not, watch one of his upgrade videos. All he shows are crude before and after frequency response measurements and that is it. You are supposed to trust him that the sound got better.
As to his upgrades making sense, they do most of the time from technical point of view. But not remotely on cost basis. He also detests EQ which can do the same thing for free. As member said, follow the money. EQ doesn't make him money so he doesn't like it. What proof he provides? He just knows that it doesn't.