Someone on Reddit who speaks italian basically confirmed that this is NOT a mistranslation. They are making this that casual (but they do say that this fireball thing was said in a vague way, but it most likely means what it means otherwise I don't know if that part would even be said).
I hope what they meant is that you can use your highest unlocked spell slots for the upcast. That it means a (1) Sorcerer and 4(druid) could use a 3rd level spell slot to upcast a 1st level sorcerer spell. THIS is madness already but not as crazy as giving fireball to a level 1 sorcerer just because he is a level 4 druid.
Your first two sentences are how it already works in 5e RAW, if I'm not mistaken. So no changes to the system would be necessary.
My concern is that Larian specifically mentioned their desire for multiclassed characters to get "strong class abilities like Fireball" earlier, which implies they're making changes to spells known, not (just) spell slots...
