I'm not even American and yet where I'm from it's common knowledge that literacy tests were used to systematically disenfranchise black people from voting.
You think at the time they openly said they were trying to take the vote from black people? Or did they say exactly what you're saying now, that they were simply trying to ensure that only smart and educated people could vote.
Your argument is valid, but isn't it clear that the status quo can't continue? Close to half of US voters decided it is OK to ignore climate change. Is it possible to educate them?
As you see in the article you linked, it was ruled unconstitutional only because it violated the 15th amendment. That amendment makes it illegal to deny the right to vote based on race, color, or previous condition of servitude. It is still legal to deny the right to vote if someone is deemed not smart enough.
The literacy tests in question were designed from the start to be ambiguous, so that the same answers could be marked wrong for a black person and correct for a white person. Frankly, there is nothing that could convince me that the Republicans wouldn't try to repeat the same thing if given the opportunity.
It's incredibly naive to think that a tool like poll tests will only be used in the narrow way you personally would like them to be used.
No matter what kind of voter you particularly would like to target, the people administering the poll tests will (ab)use them to disenfranchise whomever they please.
Even if you believe the people in power who administer your elections would do this the "right" way, if you give them power, then the next people who come along -- who you may not agree with -- will also have that power.
Keep in mind that the word "dumb" gets thrown around in all sorts of situations and has no objective, easily-verifiable test for "dumbness". It's entirely plausible that merely "making a decision I disagree with" could be construed as "dumb". Imagine if, say, Biden and Fauci had decided to disenfranchise anyone who refused the COVID vaccine as "dumb", or, worse, Trump and RFK decided that taking those vaccines made you "dumb". See how easily this can be abused?
Even with that aside, the natural consequence of disenfranchising "dumb" people is that you want to fuck them over somehow. No thanks.