If you've ever read any of my fitness diatribes, you know I agree about the stretch. Contorting yourself into that position isn't going to help you get the ROM to sit in seiza. In general, putting oneself in odd pretzel positions will not somehow magically translate into improvements in being comfortable in ordinary positions. Sitting in seiza itself is going to do the majority of the work for you, and using a structured sitting regimen is the way to go.
That stretch is of a particularly bad variety in which your bodyweight ends up passively cranking on the tissues in a position in which you cannot control the action with muscular strength. This stretch is a potential hazard for both the knees and hyperextension of the lumbar spine, and it is too dissimilar to the desired position/movement to provide the intended benefit. Passive stretching is the most overrated activity/prophylactic in the fitness world. Given proper training methods, including dynamic movement drills and active ROM/resistance exercises, static stretching is almost completely unnecessary even for serious athletes. If you insist on doing it, at least pick simple, non-contortive stretches in which you have complete muscular control.