"I only acknowledged it"

You didn't answer. You just sent "Seen, will look at this tomorrow" so they'd know it hadn't vanished. That's not working. That's manners. It takes ten seconds and it stops them wondering.

Look at what the nod says. It says that at ten past nine you read the message, understood it, and typed a reply. Everything Priya learns from a full answer, she learns from the nod. You were there. You were reading. You could be reached. The content of the message was "tomorrow", but the fact of the message was "tonight", and the fact is what teaches.

Then look at what it does for you. You've now opened the message, which means you know what's in it, which means the deck is in the room for the evening, exactly as chapter 4 described. The nod is meant to let you put the message down, and instead you've picked it up, looked at it, and told someone you'll be thinking about it, which you now are. It also invites the reply to the nod, "great, thanks, no rush", which is another buzz, and a small warm feeling, and one more lesson.

Now, the nod has a good version, and I don't want to lose it. If the window in chapter 17 is in place, and you've said it out loud as in chapter 18, then "will look tomorrow" is already known, and the nod is redundant. If the window isn't in place yet, the nod is a step towards it, and better than the full reply, because at least the content says tomorrow. So the nod is a stage, not a destination. It's what you send in the fortnight between deciding and being believed.

The honest objection is about the sender's feelings. Won't they think it's rude to get nothing until morning. Giurge and Bohns' receivers thought so, and the senders in the same studies mostly didn't. The sender who wrote "no rush" was telling you that silence until morning was fine. The sender who didn't write it was, on the evidence, usually thinking the same thing and not saying it.

The practical point is that a nod at nine and an answer at nine teach the same class. If you're going to send anything, send it at the time you want to be known for.