Probably doesn't need as much intestine as we do, so the length of that tube is sufficient. Or perhaps most of the ingestion is done in the stomach.

Or the diagram coyld be simplified for illustrative purposes, like how our human models focus on internal organs and dont often draw, say, the visceral sack they sit in, and the webbing that weaves our organs together so precisely that surgeons can jiggle us on the table to slot them back into their places.

And yeah, that is where the snail anus is...