Consider then the Others. What has been seen of them, what may be deduced?
They appear humanoid up to the limits of visibility. They breathe the same air he breathes. They speak roughly Baseline, in the sort of audio frequencies his ear is accustomed to hearing.
The Others share vast amounts of information-data-complexity with himself / dath ilan. Off the top of his head, Othrem thinks he is being fully inclusive if he categorizes possible explanations of this correlation as 'shared causal ancestry', 'convergence', and 'filtering selection'.
Their time could descend from his own.
His time does not descend from theirs, given that their attitude toward hugs has not been historically recorded.
Both times could descend from a common ancestor; a mutually past dath ilan that decided, after the construction of Baseline, to optimize more around how affectionate people were toward each other, perhaps at the expense of other qualities like a stronger dependency on notebooks to assist with thinking.
Those are the straightforward explanations, the ones in which all shared information is explained by shared ancestry.
You can suppose that the Others descend from a dath ilan ancestor from before the invention of Baseline, and explain away the shared information in their mostly-shared language by saying: there was convergence in which best solutions for language-invention were found. But unless he's missing something about language invention, surely there must have been some arbitrary choices at some point in the process... well, no, maybe they actually did graph out every sibilant and pitch and their cost to vocalize, figure out what combinations would most cheaply distinguish which basic words, in order of their expected frequency of use...
Okay, no. Othrem isn't really buying that. Unless the constructors of Baseline were explicitly like, 'Any time we break a tie randomly it reflects a failure on our part to think of something we should be optimizing...' Wait, no, actually that's pretty plausible. Othrem can absolutely imagine the language-constructors reasoning like that. Just because he can't see all the desiderata, does not mean they are not there.
Can he imagine that this is a world that diverged from dath ilan ten thousand years earlier, and they still uniquely invented Baseline?
Othrem really wants to go 'NO WAY IN SHIT' but he is not, actually, sure. Maybe there's just enough desiderata to optimize if you are actually a linguist.
However, more likely is that as you start to go further back in time to locate a point in divergence, leading to less shared ancestry, one must begin to postulate filtering selection: Othrem and the Others were selected by the process that brought them here, to share a language. This is beyond the realm of flukes and near-certainly into the realm of alien machinery and unseen intelligence; there is no quirk of physics which does that. (Frankly, there was not going to be much of a quirk of physics that materialized him naked in a cave in any case.)
And then once you permit the concept of filtering selection at all, it permits the Others to be from much further away in configuration space and maybe even from some other Reality entirely. They could have simply been selected, from across some great swathe of locally visible Reality, to happen to speak mostly-Baseline, and to breathe the air he breathes, and to speak in frequencies his ears can hear. Or vice versa, of course, or both he and they were selected according to some common filter.
Othrem thinks he is going to mostly proceed on the guess of mostly shared ancestry with dath ilan, and maybe a tiny bit of convergence where that is plausible, to explain all shared information that is visible so far. Bringing in the postulate of selection requires a whole huge chunk of assumption about the alien machinery, and throws open the possibility-space so much wider that it's harder to work with the wild possibilities inside. He will try to try to work with the narrower assumption 'correlation proceeds from shared ancestry' until it is visibly violated. He is a very average person of -0.2sd and does not actually do that great with trying to actually hold sixteen wild possibilities inside his head simultaneously.
Maybe the Others do natively better than that, and that is what the notebook is for. Othrem has not really been using the notebook so far because to start visibly recording what he is thinking, he'd have to have reasoned through possible Other Conspiracies first, or how easy they'd find it to decrypt any attempted codes he used.