Since the greater than (>) symbol is ambiguous, we can't say if treaty > supremacy. By what measurement? Player base? Supremacy clearly wins. Game design? Supremacy clearly wins, since there aren't 40 minutes of no decisions and no player interaction. But let's focus on the claim made in the OP:
Treaty is the obviously more challenging and more difficult game mode to master which both paper and reality showcase.
If we take "challenging" and "difficult" to mean the same thing (if that was not the intent I invite the OP to let me know the difference) there are 2 claims here:
1. Treaty is the more challenging game to master on paper
2. Treaty has proven to be the more challenging game to master in practice
First, we need to talk about the word "master". What does it mean to "master" a game? Is it to play the game completely flawlessly? That would be impossible either way, so we can conclude that it's not what the OP meant. Is it, then, to be the best player in the world? That seems like a bit of a high bar as well, plus no one ever managed to be the best at both games so our analysis would soon reach a dead end. Is it to be within 100 ELO of the best in the world? We could potentially work with that, though it is still problematic. The best player in Go has an ELO of 3850. The second best, 3700. So there would still be only one player who mastered it, which sounds wrong intuitively. But let's ignore that issue and say to master a game is to be within 100 ELO of the best in the world.
So let's start with
claim (1).
How do we measure how challenging something is to master on paper? Is it about how much time it takes a player with sufficient talent to get there? Or how smart one has to be to be able to do it at all? Both seem reasonable. Since the OP didn't specify, and since "the amount of time it takes" is more workable here, let's go with that.
To be within 100 ELO of the best player is to win at least 33% of your games against them. How much time would it take, on paper, to get good enough to beat the best sup player 33% of the time?
First we need to figure out which player we're talking about. Obviously not the current top players, who are trash. We would be talking about iamgrunt, who was the best player during the height of the game's competitiveness. How much time would it take, on paper, to get within 100 ELO of him at his peak?
I have no idea. No one has any idea. This is an impossible question. The same applies to treaty, of course. No one has any idea how much time it takes to get within 100 ELO of whoever the best player is/was there. It follows that the OP's claim cannot be proven or disproven, and is therefore to be considered frivolous, which in itself is some admittedly anecdotal evidence towards treaty players having low IQs but I digress.
Claim (2), then.
This one is problematic for another reason, which is that some skills needed for both games transfer between them, meaning no player in history has ever learned both of them starting with a clean slate. Either a player with treaty experience starts playing supremacy, or the other way around. For example, a treaty player comes to play sup and due to the transferable skills takes much less time to become good at it than they otherwise would have. Can we ever know how much time they would have taken had they started with a clean slate? No. Can there ever be a player who starts playing both games with a clean slate? No. Can claim (2) then ever be proven or disproven? No. Ergo, this claim is as if not more frivolous than the other.
TLDR: 0/10 post, go back to playing dead games LOL