But why can the Marklar speakers understand it? Is it subtle intonation? Or is there a hidden way they convey their nouns?

TL;DR its 2 AM and im thinking too hard about this.

If you took a slice of a sphere, cone, and cylinder, you can get 3 circles, but they are not circles, and the Marklar understand this somehow. What other dimension do they see that we dont? How are they able to convey "circle" in 3 different ways to each other, but not others.

Maybe it's where in the sentence is it stated: if Marklar is the 3rd word, then it means X. Another possibility is how many times Marklar appears in the sentence. If it's 4 times, it refers to a specific thing. A combination of these rules could also apply, but I feel that still limits the amount of possible things.

Another variable could be time. If the Marklar have an internal clock advanced from ours, the definition of Marklar could be based on when it is said.

But all that seems unreasonable since ideas arent pre-determined. Im inclined to believe that, through their superior intelligence, they communicate partially on another spectrum, whether that be light, sound, or otherwise, and are unable to communicate that the spectrum even exists to a human.

They cant tell us about their extra ordinary communication in the same way we cant communicate to an animal how light or sound works. Humans can't fathom the Marklar.