But then Mr. Weir manages to stretch credibility (mine at least)—something he hasn’t been able to do with The Martian or Artemis. Grace and Earth are not the only ones in this predicament and Grace ends up making first contact with an alien who goes by Rocky. And then I start wondering if the plausibility line has been crossed. Specifically these got my goat (Houston we have several problems!):

1. Language: I’m conditioned to believe (perhaps because of the movie Arrival?) that establishing a basis for communication with a member of an alien species is a tedious and long drawn process even when linguistic experts are involved. Rocky and Grace establish effective communication in just a few weeks with an almost bijective mapping between English words and Rocky-speak. Neither is a language expert: one of them is an engineer and the other a high school teacher turned earth savior! See this post that articulates this problem much better than I can.
2. Rocky and his planet folk are Blind: To be clear this is not a problem about Rocky not being able to see like Grace….after all humans only see a miniscule band of the entire electromagnetic spectrum—the band we are commonly awash in as sunlight. Mr. Weir explains Rocky’s blindness based on the futility of eyesight when no light reaches the surface of their planet given the dense atmosphere. There are several problems with this but the most important one is this: as this post shows, the environment on Rocky’s home cannot be much different than the bottom of earth’s deepest oceans in terms of light/darkness. And fish and sea life have evolved extremely sensitive vision to detect if nothing else, other life (prey or predator) as you can read here and here. This might be fascinating but not surprising since electromagnetic radiation is ubiquitous and any ability to sense it (not necessarily at frequencies visible to humans) offers immense evolutionary advantages. So Rocky and his kind being the most advanced life forms on their planet, yet completely blind to the entire electromagnetic spectrum is hard to believe. That planet is a hot place. At the minimum, dominant life would in all likelihood have evolved infrared sensitivity. In fact any creature that did would have a massive advantage over ones that did not. Rocky having some vision, perhaps in the IR band would have made this a lot more believable.
3. Rocky’s ‘seeing’ Grace using Echolocation: So Rocky is blind but can essentially sense objects like a bat. But this ability has presumably evolved to be accurate under his home conditions: 29 atmospheres of methane based air pressure! We know how atmospheric pressure, composition and weather changes can throw bats off (because the speed of sound changes). Yet Rocky has no problem grokking Grace and his ship where sound would have entirely different characteristics to what Rocky’s sensing organs are calibrated to! I also imagine that echolocation has greater limitations to spatial resolution than electromagnetic signals. This means, Rocky’s species is fundamentally limited in their ability to understand and control their environment without additional sensory technology. This would be completely fine if Rocky was assisted by other ‘seeing’ technology, except Rocky and his kind don’t even have basic computational technical assistance! Which brings me to:
4. Rocky and his species have no computational technology!: Yet they were able to make a trip across interstellar space! The explanation here is presumably that ‘Rockys’ are just really good at being able to do math in their head (or equivalent body part.) This is hard to imagine for the simple reason that this would have to be an evolved trait which would further imply an evolutionary advantage to computational ability. It’s hard to imagine an environment where calculating the inverse cosine of 1.3245 radians is an evolutionary advantage against other species vs. evolution hardcoding the needed response in expected behavior. E.g. a dog doesn’t do differential equations in it’s head to accurately catch a frisbee! 
5. Xenonite: This magic material and that too involving a notoriously unreactive noble gas can solve all problems and is arguably the biggest scientific liberty that Mr. Weir takes in his book. It short circuits the majority of problems that face two alien species engaging for the first time. But as he describes towards the end of this video, he does make sure to assume a tensile strength lower than the maximum theoretical possible strength of any material