Oh, so yeah, there actually is no antialiasing line, you'd have to add it in yourself. At the bottom of the [grim-win] settings part add this:
You can replace the 4 with whatever, last time I used it, I had to insert a really high number before I could appreciate the difference (I think it was 16, although I'm not sure). Just make sure to use even numbers, since I don't know if rounding off odd ones was implemented.
Now it should look something like this:
NOTE: This is not how an actual configuration file looks like, it's just for reference.