Real people are inconsistent -- true. Can anyone forget that they are afraid of snakes, allergic to bee stings or over forty? I can't. However, inconsistent dialogue and behavior can be a good thing when --
Inconsistency is part of the character - and as long as it's clear that inconsistency is part of the character's composition. Don't have an animal rights demonstrator wear a fur coat and dismiss that as a quirk in her character without prior or followup justification. Maybe she's a flake and doesn't make the connection or she's a wannabe, poser, or apprentice activist.
Inconsistency is explained away later - the coat is a fake or she thinks it is or maybe it's a sentimental gift from a dead relative or she wears it out of revenge because rabid minks killed her poodle or she made it out of the beloved poodle when it died of rabies.
Inconsistency is used as a subtle reveal - maybe not every reader or viewer will catch it but later on, they can look back and say "ahhh, that should have clue'd me in".
Inconsistency is used an an obvious reveal - we need the audience to learn something before the other characters, the inconsistency triggers the big reveal or is the big reveal.
Inconsistency is a red herring - either on the part of a character or the writer.
Inconsistency is a deliberate unanswered question - In Pirates of the Caribbean, Mr. Gibbs curses pirates at the beginning of the film but ten years later is Jack Sparrow's go-to man to get a pirate crew. We're never told what makes Gibbs jump sides but we're given clues -- we know he has a drinking issue and doesn't appear to be overly fond of Norrington. But it works. Why? Because it's designed to.Sometimes, inconsistency is not something the writer can control. Maybe the director or producer insists on changes during production and writer rebuttals are disregarded. All kinds of things happen once other people put their fingerprints on the script. But purposeful inconsistency can be a good thing.
So perhaps McKee is not opposed to orchestrated inconsistency and maybe, if we asked him, he'd object only to unintentional or sloppy inconsistencies.
Then again, I could be wrong. Sal just said that I'm not a day over thirty. And, am I really afraid of snakes?