Do you mean large screen support? Currently there is no way to force it on. The app measures the screen spaces and determines if multiple columns is appropriate. If you want this feature let me know and I can add it to the to do list.
It really could just be that I'm an idiot and can't remember correctly. I'm putting together a small overview of lemmy apps for the family website, and was comparing features, and could have sworn summit was one of a few that could do that.