Yes, the venue or the speaker matters, not just the message.
Calling that an appeal to authority, or failing to keep an open mind, is just something the quack cries to explain their lack of impact. That cry is just another thing it is entirely reasonable and defensible to utterly ignore.
As jjgreen points out (https://news.ycombinator.com/item?id=34937266), it does.
But, whether or not your point of view is legitimate—in my case, I agree with your point of view, and won't give serious consideration to anything only available on vixra, and even just finding out an otherwise reputable-seeming paper was on vixra would give me pause—it is literally credentialism.
I think it's good enough to rely on the fact that the world has a lot of people in it, and there are more than enough people, not even including the idiots and misguided, but more than enough provably rational people who just have decided it's their hobby or even their vocation, to investigate even wacko claims, purely for academic reasons without necessarily caring about the claims themselves. They will bring anything worth our attention to us. Maybe we are all one of those people on occasion or on specific topics. To me that is quite sufficient way to allow for the possibility that someone did actually discover a way to burn water essentially like gasoline.
We should be clear though it's the narrowing process that has the value. Credentialism is just one possible means of narrowing that scores highly on ease of use, but perhaps not so highly for learning radically new things.
The internet is absolutely full of nonsense, and nobody is obligated to look through it in the name of open-mindedness. Most of us have things to do.
Mind you, I'm not saying the paper is bad. I'm saying it needs to pass certain filters before I spend time giving it a try.
It's also a useful bit of information when making the decision what to spend time on. Obviously this is a decision that you have to make before reading the paper.
If you're a bit familiar with typical physics crackpottery there are many other red flags that might tell you this is not worth your time.