This reads like an exercise in deliberately misunderstanding the word adversary.
If you know more than others, that's great, but in that case please share some of what you know, so the rest of us can learn.
Cheaters in games like Magic are very rare; if most people tried to cheat whenever they thought they could get away with it, we'd be forced to set up competitions with more stringent verifiable rules like (off the top of my head) "all cards must be drawn and given to you from your deck by the opponent". We haven't done that, so I infer that most people don't try to cheat.
(In your favour, I do concede that everyone writes down their individual understanding of the history of a given chess game; but there are weak instrumental reasons for that even if people aren't cheating, because it is possible to upset a board by accident.)
There's also the aspect of how hard people are actually competing. For a casual game like MtG, people are mostly there for the fun of it, not to win the competition.
For a deal worth millions of dollars, people are more likely to lie and cheat.
The number of people who declare they can totally trust what an adversary says because they agree with it is astounding, as though a committed opponent wouldn't do anything if it gained advantage including feinting in a way which seems unadvantegeous to gain long term advantage.
In the end, the only winning move is not to play. If you believe an adversary said something (or "a liar" if you prefer), you ignore it entirely. You make your mind up about what you believe based on evidence, and you decide if you agree with someone based on how well their statement comports with the evidence.
Naturally people will try to fabricate evidence, and even good faith evidence may be unreliable, so you'll have to do your best to access it's veracity. But what the adversary believes or appears to believe is largely immaterial.