I think an unreasonable amount about what Xeno's extremely short description of Senku to Stanley to identify him to shoot means.
Mainly because he says Senku is 19.
But Senku isn't actually 19. He's 18.
Stan and Xeno have had their 4th birthdays after reviving already, but Senku's birthday is...earlier in the year, but because the birthday year starts in April, Senku's 4th birthday after revival hasn't quite happened yet; they're still about two months away.
So...Xeno doesn't know when Senku's birthday is, I guess.
But Xeno is still kidnapped when Senku's 19th birthday actually happens, and you know that even their strike team isn't going to let Senku's birthday pass without acknowledgment whatever the situation. So I'm guessing he knows when it is now.
And that's gotta have been a deeply weird experience. Especially as Xeno did not know exact dates or years when he revived but was making a guesstimation, so now it's time to realize the horrifying method via which the KoS has an accurate calendar (I do think Xeno would have been able to cobble one together with knowledge of precession and constellation shifts, but it would have taken him a lot longer and I'm not sure he'd have ever quite gotten exact dates down).
But another, I think hidden, detail is:
Xeno knows how long Senku has been awake.
The series did this whole very cheesy very emotional very silly moment where they were petrified for the exact same amount of time and woke at the exact same date because the stone is worn away primarily by Brain Power and it indicates their intelligence/brain usage. So of course these two woke up first, and together, because they're Equals.
Xeno is not unaware of people waking up later, even without having met Chelsea yet. They put up a sign about it.
And Xeno just Knows Senku has been awake at least as long as he has, or really, exactly as long he has.
He really believes in and respects the hell out of Senku. Maybe just as much as Byakuya does. Xeno was too busy to think about Senku, and he didn't immediately assume Japan was Senku, which I think he really could have. But barring that...yeah, I dunno, their relationship makes me emo, okay?