The important part of "Fireball-chucking D&D wizard" was "D&D Fireball". As in, large fireballs that roast lots of people at once, not dinky little things comparable to a pistol shot. Later on in the paragraph I elaborated on the concept of discretized, resource-based magic (spell points/slots) which scales based on a uniform resource ("rounds per level").
Nod. And some of that is just getting into emulating a mechanic, not a concept.
Like I said, I'm not a fan of GURPS and haven't kept up with it too closely. I thought that some of the later stuff presented more on the fireball front than the dinky ones that, like you, I remember from it's first take on magic back in, what as it, '86?
Now, with an effects-based system, like Hero, you can even emulate the mechanical quirks of another system. So you can have the straight-up 20'r fireball /memorizing/ magic-user, as 'discretized' as you liked - if you wanted one for some reason.
The point though isn't that a classless system lets you get precisely the same result as a class system, but that it lets you build-to-concept - any concept - more readily than a class based one. For instance, turn your example around: can you build a c1986 GURPS:Magic mage using D&D? No.
'Wizard who works exactly like a D&D wizard' /is/ one concept out of the millions a player might come up with. The only concept of you can do with a wizard in D&D, one that you can't do, precisely in GURPS, though you can do dozens of others GURPS:Magic might be able to do, that you couldn't in D&D, and one of thousands you could do in Hero.
Class systems are just restrictive. Some, like classic D&D, more restrictive than others, like 3.5 D&D, but all restrictive.