Should I tell a superior [in a meeting] their statement is incorrect?
(I will not focus on how to prevent the situation, as this is not the question. But of course preventing a bad situation is better than trying to fix it.)
Let's try to approach this one methodically. Why would your superior make an incorrect statement?
He does not know better.
Does it matter and will be result be unalterable when the meeting is over? The answer is usually "No.". Any decision can be revoked due to "new" information. So it is sufficient to update your superior with your information after the meeting. If the answer is actually "Yes.", then politely state your information. The listeners will realize themselves that your statement contradicts your superior, no need to rub everyone's nose in. If nobody would want your input, you wouldn't be in the meeting.
He does know better.
If he does know better, yet states something else, it luckily doesn't really matter why he does it. He has a reason and your public correction will go against his agenda, no matter if his agenda is a good or bad one. He believes so much in his agenda, that he does not mind to be flexible with the truth. In such a case, correcting your superior means he will be pissed off and you gained nothing.
If you update the superior after the meeting with your information and he does know better, nothing bad will happen. He might tell you the reason, he might pretend that he didn't know better but that it is "not important" enough to revoke the decision now or he might tell you to care about your own stuff.
So, reviewing all the possible paths, it's easy to see that you should not say anything in the meeting, unless absolutely necessary and always update the superior with your information, because no matter what the reason was, it will end up the best way - if you couldn't prevent the situation.
As a side note, if your superior states something incorrect or dumb in a meeting, you can just raise your eyebrows (this of course requires the right seat). If they are good and it is the first case scenario, they will catch this and continue with something like:"but I'm no expert on this. What is your opinion?"
dashed my "improved SEO" reasons for replacing the company website
This is actually quite something different. You said "X" and he said you are wrong. It's basically the reverse situation. I think you should defend your position within reasonable limits:"Yes, you are right, it's not possible to manipulate them, but it's possible to optimize the content so they can better grasp what the website is about and therefore navigate the search engine users better." The big problem is, you usually don't have a good comeback when you need it, unless you are well-prepared for the objection. There is no better way to handle that situation than you did. Evaluate how important it actually is to insist that you are right and then follow your instinct. You can also just state that you disagree and go on:"Well, I beg to differ, but as there are other good reasons, we should focus on them then."
Okay, I violate my first statement and write something about how to prevent it:
If you expect objections, not only prepare counter-arguments, but add them to your presentation or lecture! Nobody can state that SEO is hocus pocus, if you have a bullet point:"Why some SEO is hocus pocus and some not".