Someone who says “Nobody is stupid, they’re just ignorant” is saying something stupid. They could be stupid for not thinking about what they are saying or they could be ignorant for not differentiating between ignorant and stupid, or they could be both ignorant and stupid for not looking up the meaning’s of the words ignorant and stupid.
Stupidity and ignorance are completely different things.
The word stupid is an adjective used to describe someone with a slow mind. The word ignorant is an adjective used to describe someone who is lacking in knowledge. Being stupid isn’t being doomed to be ignorant, think book smart and being unable to apply knowledge. Being ignorant isn’t being doomed to be stupid, it’s just being doomed to look stupid for not checking your facts.