As with showing and telling, I think this is good advice that shouldn’t be made into a universal rule. Fiction can reveal other things than character.
Setting, for example. Even in non-genre fiction, there can be a real pleasure in getting to know an interesting place for its own sake. Much more so in historical fiction, and most of all in sci-fi, horror, or fantasy, where exploring an unfamiliar world can be a major part of the appeal. Gormenghast’s setting has as much character as any of its characters; some of its best sequences are all about the castle rather than its denizens. China Mieville rolls out a new imaginative setting or creature every chapter, most of which arguably ornament the plot rather than advancing it. And while Mieville has said some famously unkind things about Tolkien, he also recognizes “subcreation”–the elevation of worldbuilding to be just as important as plot–as one of Tolkien’s key literary contributions.
A sentence may also be there to express a theme or idea. Yes, those are often best expressed through plot or character (or setting). But they aren’t always. I’ve read terrific novels by Julian Barnes and Milan Kundera where digressions into ideas are some of the most memorable bits. If the novel is epistolary, like Marilynne Robinson’s extraordinary Gilead, the author doesn’t even need to go omniscient or break the fourth wall to use sentences this way.
So yes, I’m another who’d