So I do suppose a bit rides on how far into the speech OP got--since OP said they had only just begun

I think you might've missed what I said in that paragraph. My interpretation is that OP began their spiel but hadn't actually gotten to the part where they fired him but the dude knew where the conversation was going.

It's entirely possible (especially in a small company) that they knew that someone was going to be fired and when he went in and saw the somber tone he already knew it was him before a word was said or he knew based on how OP started. Ex: "These things are never easy..."
I've worked in small and big companies and, honestly, we've almost always known when people were about to be let go and when the person has gotten called into

At the end of the day--we're both interpreting it differently, which I already owned and that's why I said it really depends on how much of the firing stuff got out.
Had OP already said he was being let go in some way or did they never get to it.
Both options are entirely possible and, IMO, it makes a difference how far into it OP got.