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.
