What is being proposed in this video is a simple process (trick) that allows you to quickly find out what the internal volume of the box is, which will give a result similar to that obtained by the Thiele-Small method.
From a simple formula, you can create a table that allows you to quickly size a speaker box, leaving the calculation of the tuning duct and the possible use of internal sound absorption lining for a second step.
Speakers are designed based on the woofer (bass speaker) that you intend to use. From this woofer, you can determine whether the box will be an acoustic suspension type (sealed box) or a tuned duct (bass reflex). The next decision is to find out what the internal volume (literage) of the box will be. This is where the calculations based on the Thiele-Small parameters come in, either using software or doing the math by hand, based on the formulas.