So, you mean that if you multiclass 1 level into wizard, that youcould only use those learned spell with the wizard spell slots, right?
Yes exactly. you can cast the Wizard spells you learned, and you can learn any level spell you can cast as a Sorcerer - so you could have 2 levels in Wizard, learn haste from a scroll (level 3) and then you can cast that with a Sorcerer level 3 spell slot AND you can Twin the spell.
Totally broken.