I've been very mildly involved in this project, so I can give my two cents. While it's true that structural / type checks are not difficult to implement, there's no real need for a back-and-forth when you do static checks -- you either fail out, or run rules to fix.
There's something a bit different that we (should) expect with LLMs (and FMs more generally) since they are fundamentally interactive, so you can actually get them to correct things in interesting ways. Passing the outputs of static checkers back to the models is one nice trick. I (and some friends) have been exploring some stuff with using models in the loop for evaluation (more research side), and I think guardrails is directionally exciting in bringing that kind of vision into more production type settings. There's also just the crud of dealing with LLM code...
There's something a bit different that we (should) expect with LLMs (and FMs more generally) since they are fundamentally interactive, so you can actually get them to correct things in interesting ways. Passing the outputs of static checkers back to the models is one nice trick. I (and some friends) have been exploring some stuff with using models in the loop for evaluation (more research side), and I think guardrails is directionally exciting in bringing that kind of vision into more production type settings. There's also just the crud of dealing with LLM code...