I think his point is that whatever background you're likely to come up with this mess is going to end up looking like a pile of s.hit.
Why? Because it doesn't fit into one of the prescribed descriptions of a character class?
I think it's silly to assume that the people in the D&D world think of themselves (and each other) as strictly bards or rogues or even paladins. Every person you know in the real world has a broad range of interests and abilities beyond what they studied in college (if they even went to college). Scott Adams was a programmer, budget analyst and project manager before he started drawing
Dilbert. Is he a "mess"?
It's not so strange to me to think that a character with some inherent magical ability (sorcerer) might choose to augment that by forming a pact with an arcane patron (warlock), and eventually develop a level of devotion to that patron that resembles religious zeal (paladin).
I think the reasons some people feel a disconnect about multiclassing is because they think of gaining levels as a discrete process rather than a gradual blossoming of power. I see the leveling-up process as a continuous path, not a series of steps, and that path ultimately leads to the fully realized character concept that presents itself at level 20.
That final realization doesn't necessarily fit into the cubbyhole of one of the classes in the book, but that doesn't make it a "mess." It just means that character had a broad range of life experiences and interests.
So your warlock is in the desert and suddenly gains a level of paladin ... so what? It doesn't mean he one day decided to open his heart to Pelor and suddenly gained all of these powers. It means he's been thinking about Pelor for a while now, and also paying attention to his fighting skills, and this moment in the desert is when it all finally "clicks."
It makes no less sense than suddenly being able to cast more spells or make more melee attacks than you could the day before. You accept the continuous-growth explanation for single-classed characters (and most DMs don't require them to have trainers), so I think it's a little odd that people make such heavy weather of it when the new level happens to involve a different label than the previous ones.