That other child probably doesn't really know what autism is. I would take that exchange with a grain of salt. Just like if your son said "I have a unicorn" and the other child said "No you don't".
That child might have a hard time when other children get attention. They might have met one other child with autism and don't understand that its a spectrum. They might think it means "artistic" but not have a very high opinion of your child's art skills.
Really it could be anything. Your child is going to have a lot of these interactions throughout his life and he's going to develop his own way to deal with it, to advocate for himself.
I wouldn't step in these cases, maybe just watch what happens and talk about it with your son later if he wants to.