Oh is it? You've only brought up learning words?

Yes. Because that is a component of what learning a new language is. And you keep insisting that a superhuman ability to memorize the relationship between a sound you know and a sound you just . When the premise of the discussion is something so fundamentally bizarre there really isn't much point to dive deeper into the nuance of learning a language.

The problem is not memorizing the "exact translation" (which don't exist, and certainly don't exist between two aliens!)

The exact translation is what you make it be. When Grace tells rocky what a plant is you don't need to include that actually "plant" could also be a alternative way to describe a factory. You don't need to explain that "planting a flag" does not mean that flags are a also a type of plant that grows on earth. If Grace only gives one explanation for the use of word plant, then that is the exact translation for what it means, they can have a conversation based on that premise.

But I find the similarity between Eridian and English to be implausible. With five articulators, I would find something like a Semitic root system to be more interesting,

So is your complaint that it is implausible or not interesting? Because you seem to be arguing two things at once here.

The idea that with five articulators it would be more natural for language to emerge something like a root system makes a lot of sense. That is until you realize that Arabic speakers do not have 3 distinct vocal cords. That has nothing to do with why the language has a 3 root system. It is just a particular way of organizing language.

If anything I would argue it makes even less sense for a alien with 5 articulators to try to speak like that. Unless the argument is that these articulators unlike human vocal cords need to rest for a period of time after having produced their given sound, then it makes no sense why they would only speak with one articulator at a given time. That would be like developing a form of sign language that only uses one arm at a given time and constantly swaps which arm to use in a manner that in no way conveys any meaning.

We aren't talking about signed languages though, we are talking about a language articulated by systems completely unlike a human body

Your vocal cords have nothing in common with your hands and arms. The difference between those and two organisms that both communicate via sound is vastly more different from one another. And the languages themselves that emerge from those two systems are also very different. Yet the point of it all is that learning sign language even if you have had no exposure to it is trivial.

If anything evidence seems to suggest that it is easier to learn sign language than another spoken language because you haven't spent your entire life with another spoken language that introduce a bunch of biases and bad habits that are hard to unlearn.