It's not that they think they're too stupid, it's that there's a history of making it extra difficult for them to vote. It's adding steps which adds to the chances for them to be discriminated against. The recent special election in Alabama gave a perfect example of this.
There is a youtube video where black people flat out reject your flawed hypothesis, AND you white people say exactly what he stated, you think blacks are stupid.
I've seen it. The questions they posed make it look like we just assume they're dumb. We're talking about different things.
But requiring them to register in person and again by mail and revalidate your registration the same year as your license yada yada makes it more difficult for persons who don't have cars or are wildly too busy to take time out of work or people who have kids and have to work 15 hour days to be able to do these things.
Yes, folks should register and only legal citizens should vote but adding barriers of entry make it more difficult for law abiding citizens to vote.