If you aim to be "human-friendly" (and that is, as I understand, the raison d'etre for YAML), there is a subtle semantic difference between "true" and "on" (and "false" and "off") and as a human it may be nice to express that semantic difference.
As for that semantic difference, if we expect the light source to have one of exactly two states (that is, "not a dimmable light"), we probably want to express that as "lightsource: on" rather than "lightsource: true".
And that is where the friction between "humanfriendly" and "computer-friendly" starts being problematic. Computer-to-computer protocols should be painfully strict and non-ambiguous, human-to-computer should be as adapted as they can to humans, erring on "expressive" rather than "strict".
I am also not sure if I am happy or sad that the set of configuration languages in the original article didn't include Flabbergast[1], which was heavily inspired by what may be simultaneously the best and worst configuration language I have seen, BCL (a language that I once was very relieved to never have to see again, and nine months later missed so INCREDIBLY much, because all the other ones are sadly more horrible).
[1] https://www.flabbergast.org/