He is not making a perfect miter cut, he is making a miter cut that looks perfect only from the face. We call it a face fit. Miter cuts are butt joints. The best hope you have of keeping them tight and together over the longterm is for them to fit perfectly through their entire thickness for maximum glue surface. Back beveling a cut is an old carpenters trick, often used because surfaces you fit to are seldom even, flat and straight. Back cutting isn't a big deal for applications like door and window casings and other carpentry work but it isn't something that goes with finer quality woodworking. If you are coping crown molding, that is a back or under cut and coping would typically be used when installing crown in a room. Crown on a piece of furniture is not traditionally coped.
If you looked closely at the cut he was making, the piece he put under the casing kept the casing off the table and the casing actually got pushed down when the saw made the cut. That can be dangerous. If he was cutting a wider board or a longer one in the middle, it may cause pinching on the blade. That's when the saw binds, may jump or charge forward. It can also cause burning or just give a poor quality cut. A very large percentage of Youtube videos on woodworking should be taken with a grain of salt. There are some rather uniformed and dangerous folks posting videos. Another safety violation was that he did not keep the saw down until it stopped. No safety glasses and poor control on the work piece that could move like a teeter totter. Don't take lessons from this guy.