From the magenta tint, I should have guessed that I had missed the green channel. The tutorial mentions:
To create a new channel that will become your “Lights” channel, drag any one of the RGB channels down to the lower window (it doesn’t matter which - they all have the same data due to the desaturation operation).
By moving the channel down, it had become unselected. So the green channel had stayed the same, red and blue had become brighter, and red and blue make magenta (magenta is the opposite of green in the chromaticity diagram):
You can select channels with the Brightness/Contrast dialog open. Selecting all channels produces an image with similar color that is much brighter: