From the podcasts (I'm a big fan and have listened to all up to the latest one) Carroters believes in the conventional ideas about poker psychology, which I know MM doesn't.
I would like to see an experiment, where players in a cash game record their starting stack and whether they consider themselves to be playing their A, B or C game. If they feel that has changed (or was a mis-estimate) they can change at the end of a hand and record their current stack, but can't post hoc change the "level" of their game for a hand that's finished.
When people make an unconventional river call and are right they think "I'm on my A game" and when it's wrong they think "Why did I call, I'm on my C game", but that's just results oriented. An experiment would tell us whether the ABC game is a real thing (and how much effect it has), but conducting one seems not to be necessary to poker psychologists as poker psychology is a pseudoscience.
Having said that, I don't think we should criticise the book based on the fact that Carroters has drunk the same Kool Aid as almost every other single person in poker. For example I also highly recommend the work of Andrew Brokos who writes for the 2p2 magazine and also has a great podcast, but he also talks about the ABC game - though like everyone else not in hand analysis.