Yes. Multiple people can come up with the same theory independent from each other. Every now and then you'll see a theory the creator says "I just thought of" and when you point out that their theory has been made before, they are surprised. In Alpha's case, its very suspicious. As I said before, a lot of his theories are supposed rip offs of OJ theories as they are the same with minor tweaks and Alpha has an OJ account as well and he has commented on the theories that he ripped off. You saw the Gorosei one and guess what? There is more.I don't watch YT theories, hearing people talk about theories like that annoys the hell out of me. Even more so when you can't say anything back and holding 'discussions' in the comments section there, yeah no. Most of the time they're also way too 'positive' about their theories, which annoys me even more. It happened a few times that someone posted a theory from such a vid and that everyone is like "amazing" and I am "what the hell is this?" because they forsake the basic principles of theory making. So I don't really know about all these people.
Most theories though are of a level that they can easily be made up by different people independently from one another, so crediting others becomes rather stupid there. Though that has as a result that whenever someone does make a very detailed theory, it becomes easy to track it to its source. I probably didn't read most of these theories, but a sentence like "Uranus is the extension of life, vigor, power and prime" is not a sentence that's going be thought of by multiple people by chance, there's a connection there.
It does matter and by making vids the creator is implying he made them. If he didn't or only partially, he needs to credit the original creator or in the least the source he got it from. That's how a theory of any kind becomes credible.
Saying that he never said it was his, is plainly nonsensical. It's that this is the internet and a manga theory, if this was the real world and a more serious matter, he would be committing plagiarism.
I dislike YT theories as well as I much prefer typed theories that I can read. A lot of YT theorist think their thoughts are accurate when its just a prediction of something that hasn't been proven like uh...a theory. Just list your sources and accept the fact that your theory can be complete BS. No reason to fear being wrong and be sensitive.