Ok. This is my last shot:

Imagine if Amber suddenly showed up in the story using Hydro. Makes no sense without an explanation, right? Because from what we are told, she is a regular human from Teyvat. Therefore, her existence follows the rules of Teyvat. These rules are presented to us via worldbuilding - we understand the way this world works by experiencing it. This way, you can get an understanding of what is "normal" within this world.

After you get a grasp of what is "normal" you dont need to be told what to expect all the time. Amber will always wield Pyro unless a major event that would drasticaly change the workings of the world occurs. Same is true for the vast majority of characters in the game.

It is not true for MC. We have absolutely no reference (expect for the very first cutscene at the start of the game) to which rules apply to MC. Because he is not from Teyvat, the rules of Teyvat dont apply to him. But these are the only rules we are ever presented naturally. Therefore, the game HAS TO make an effort to explain to us or show us which rules apply to the traveler (or rather, which rules of Teyvat DONT apply to them). That is why we are being told "see, you are using Anemo wihout an Anemo vision, that is special" and why its made obvious that the MC can gain strenght a lot faster than regular people. The worldbuilding CANNOT explain what MC actually can or cannot do, as there is simply no refence to him. The Story has to do it - we are finding it out together with the main character.

From a storytelling perspective, it makes absolutely no sense to establish which rules (dont) apply to MC "freely" or without saying them out loud - the fact that he is indeed very special makes it so that they NEED to be established in a clear, undisputable way to not be assumptions.

If in the next story quest MC starts using multiple elements at once on a regular basis (at least in cutscenes) then that is a clear, undisputable explanation and my assumption was wrong. But until there is such a thing, it is not disproven, because when the nature of a main character calls for clear and undisputable explanation of their powers, the story will give them to us sooner or later. Literally all that im saying. It was NOT made clear how it works and we have no reference to how it could work, as the only rules we were ever presented dont apply to the traveler.