That one seems really strange. Particularly since there are rules in the core books about re-imaging extraneous other weapons into things on the chart (the example given having nunchaku that act mechanically the same as a club). They could have included trident as an alternate take on a spear (certainly well within the level of similarity as nunchaku->club) right there in that text field. I mean, I get why they don't do that with something like a blowgun (what is that going to be a re-imaged version of?), and also why they want rules for tridents (predominantly because of a few iconic magic items and any character with a Poseidon/Poseidon-worshiper motif). That would have saved them tons of people griping about the trident being mechanically a spear, but worse.