So, what, it's OK to have traps in the game rules for new players/roleplayers because the DM can simply fiat new mechanics to help them out1? If so, that's true of literally any RPG ever, with any possible bad mechanic.
Er, that's not what was said. Never was said that the GM should fiat. What was said was that if you spend years in game practising at magic, the GM should make Dungeon World feel real and give you the benefit of what you've done. Just because you haven't spent an advance on it you can't car spells regardless of the fact that you've been studying for years? That seems odd to me.
Just because you don't like the mechanic doesn't mean it is bad, nor a "trap".
If the rule for multiclass moves leads people to ignore it because they don't like the outcome it produces, as you suggest people can do, then it is a failure of game design. What you say does not in any way support the mechanic as good design. All you're saying is that it's failure can only be so bad. And "so bad" is as bad as it can ever get. In fairness, I'll note that what you say does not in anyway imply that the mechanic is bad, but I've already presented, other, sufficient arguments in support of that position.
I won't speak for others here, but I really don't think you've given any compelling arguments that the mechanic is bad, just that the mechanic doesn't do something you want it to, and are unhappy that the response from others isn't to agree with you, but rather to say "if you don't like it, don't run it that way."
Anyway, the difference between the multiclass moves and gaining the moves via descriptive, fiction first, positioning is exactly that: one way you get to prescriptively say, "I've learned how to cast spells," and the other way you say, "I'm going to practice magic junk every day with the wizard," and through a lot of plot and description and probably more than a few moves you eventually learn how to do magic. Maybe you get the Cast A Spell move with associated stuff, maybe you get a custom move related to hedge magic. Depends on the fiction. It's not demonstrably folk ignoring the multiclass rules, it's folk approaching the same goal differently. That's not a bad thing.
(edit: removed reference to Oberoni crap. After reading more posts on it than I care to ever again, it appears nobody knows what it actually means and it's just a catchphrase to not actually mention any formal fallacy. Will now never mention Oberoni again, and if you want to discuss formal fallacies, actually discuss formal fallacies. Otherwise I will assume you are using a bunk appeal to authority, which while not a formal fallacy still leads to a weaker argument when your sources stuck are uncited)