Are you arguing about what the regs say, or about the "reasonableness" of the regs?

I would say that if you are demoing to a student for any time other than a tiny portion of the flight (less than a tenth of an hour) TO SOMEONE THAT IS ALREADY RATED IN THE AIRCRAFT, then:

1. Technically, according to the regs, he can't log the entire flight time as PIC.
2. If you are doing a demo to a rated pilot, of any maneuver that takes more than about 30 seconds, that's pretty unusual.

We aren't logging time to the nearest second, so the usual practice of "rounding" the rated student's "sole manipulator" time to the nearest tenth is reasonable.