I don't think it was just a smile. Remember how Kenshin couldn't predict anything he did? That's because all he could feel from him was joy. (although some argue it's contentment rather than joy. Take your pick)
That's true that Kenshin couldn't feel anything else from him, but I don't think that means he wasn't holding his other emotions back. Because he did eventually let them out (or at least anger/frustration)…so he must have been surpressing them or something. So therefore, I still think he fits into the "holding back emotions" character type. Which is what I was trying to say about "emotionless characters" but I lack a better term.
Unless he suddenly developed them during the fight somehow.
But still, whatever the case, it doesn't change the fact that Soujiro is an example of the emotionless/expressionless/single-expression character that end up expressing their emotions (or different emotions other than the one they usually express). Just like there are all kinds of "cool guy" characters, there's different kinds of them too. Or at least I have always grouped characters together in that way.
I didn't mean to start any kind of debate about the specific details. I just wanted to point out that Soujiro, Sai, and Mr. Grimmer are similar character types (in the general sense). That's all.