To those wondering why the crew doesn't report back: isn't it even more implausible that two alien civilizations exchange petabytes of information about themselves, translate each other's languages, and start a philosophical discussion within hours of contacting each other, as opposed to, oh I don't know, almost anything else? This is a stage for some archetypes to discuss some ethical point, and you can assume that they will think of everything that Eliezer wants to cover (or else appropriate actors will appear at the right time). That's all that matters.
Implausibilities abound, but whether or not they matter depends on one's assumptions about the goals of the story. Given that I'm not yet sure what Eliezer's goals are, I come up with my own. (Actually, I would do this anyway). I can't help but wonder what it would be like to be a Babyeater, and I ask all sorts of questions that may be completely irrelevant to Eliezer's purpose. For example: How could they not be able to distinguish between the concept of good and the concept of baby eating if they understand that survival is good and that they survived before agriculture and before they started eating babies? When are babies taught that baby eating is synonymous with good? Do babies realize what will happen to them? We know that they do not want to be eaten, so do any of them try to make reasoned arguments about why eating them is unnecessary? Have any of them tried to organize the babies in revolt? I understand that they are young and so presumably not very capable, but still. How are the survivors selected? Does the evolutionary process the Babyeaters like so much optimize for Babyeaters who run fast, or are promising babies selected early and separated from the rest of the group? If the latter, what are the criteria? How do the dynamics of Babyeater society change with their ability to detect cheaters? What are their reasons for thinking that they are good at it? Etc. etc. Are any of these questions relevant? It depends.
The point is not that Babyeaters are implausible, it's that Babyeaters are fascinating enough to think about in some detail, that doing so will eventually raise questions of plausibility, and that such questions are likely to be unconstrained by their relevance the the author's point if 1) it's not yet obvious what the author's point is, and/or 2) they author's point is, or becomes, relatively less interesting than questions about Babyeater society and evolution. After all, p > 0 where p is the probability that Eliezer's real goal is to make OvercomingBias the number one search result for "baby eating." (The previous sentence is a joke.)
TL;DR: Properly applying the MST3K Mantra is non-trivial.