I didn't watch the video, but in general: You don't want to heat the board too fast, and you want to ramp up the board temperature evenly. I fail to see how preheating the oven helps in that. Sounds to me that this promotes fast, uneven heating of the board, where sections of the board that are closer to hot parts of the oven heat far faster than parts of the board protected by heavy components. This is a recipe for microfractures on the board, which are a nightmare to find and debug.
Correct pre-heating and soaking is also essential for the paste to operate as designed; look at your paste manufactures data sheet. Sounds to me that putting a board in an already hot oven more or less guarantees out of spec conditions for the paste, at least in some areas of the board.
Summary: At first hearing, sounds like a rather bad idea. I'd like to hear the motivation behind it.