We have a few advantages not taken into account by the author. First and main one is the way the MOSFET CCS is used. The amplifier valve still sees a constant current. The load current is not taken from the regulated amount. If there is no variation in current the LED becomes more of a voltage ref instead of having to deal with varying current.

We are also running with fairly small signal, so that admonition is accounted for.

Besides, this ref agrees with my own research and has little if any internal contradiction. Some of the others( even some authored by folks revered almost to Godhood) have not been so clean.
regards,
Douglas