[synu]

Credibility of evidence is not totally subjective. I (and you, and anyone) can personally prove in a variety of ways that the earth is not flat, but still many people believe there is a conspiracy that the earth is round.

Of course they all tell each other that all facts are subjective anyway and wouldn’t it be cool and interesting if you were in on a big secret and everyone else except you and your new friends were sheep.. oh and by the way did you know that it’s actually lizard people who started the idea that the earth was round? Doesn’t that feel right and would explain why your life hasn’t turned out the way you wanted? And so on.

In this case I’m explicitly talking about making up non-credible evidence. If everything is subjective then whatever I made up is as good as anything else, I just need to tune it to your biases to make it feel as truthy as possible. But if you believe me then you’ll start to have problems when you need things to actually be true (like running a space program or military for example).